Demo Videos

Before I get into some of the more technical details, I want to show some video captures of the emulator running. And if you happen to find yourself thinking that it looks like something you’d want to play with, I’m releasing it all as open source. There will be some links later in the post where you can download the emulator, and for the ambitious, all the files needed to build your own hardware ApOPL3xy are available as well.

VGM Player

One of the most iconic pieces composed for the OPL3 is “At Doom’s Gate,” the music for the first level (Episode 1 Mission1, or E1M1) of Doom. vgmrips.net has the VGM capture of this song available for download, and I can’t think of a better song to demonstrate the VGM player feature. Note that most, if not all, VGM files only use the first two output channels, since no 4-channel OPL3 sound cards were commonly available.

The numbered buttons below the display correspond to the colored buttons on the hardware, and the two groups of three buttons, labeled ↺, ↻, and ☟, represent the rotary encoders being turned counter-clockwise, turned clockwise, and pressed like a button, respectively. At some point, I may improve these to be a more skeuomorphic control, but the buttons get the job done.

MIDI Player

My last post showed the new MIDI file player feature on the hardware ApOPL3xy, but due to difficulties filming the backlit LCD screen, it wasn’t easy to see what was on the display. The next video shows the emulator playing a MIDI version of the Doctor Who theme music. As before, I arbitrarily assigned MIDI channels to output channels to make the VU meters a bit more interesting.

Loop Controls

Between the last post and this one, I added loop controls to both the VGM and MIDI players. It’s a small feature, but potentially useful. The loop indicator is just to the left of the total time in the player interface. It starts out as a right-pointing arrow, meaning no loop. The other loop modes are single loop, represented by an arrow that curves from the right to the left, and continual loop, represented by two arrows pointing at each other’s tail. The 5 button cycles through these modes.

This video demonstrates the single loop mode being used for a MIDI file with a version of the Pac-Man music. It also shows the page down menu navigation function, which is mapped to button 9.

MIDI Input

My primary motivation for building the ApOPL3xy was to enable MIDI input to control an OPL3 chip. So far, I’ve demonstrated playback functionality, but I want to show the live MIDI capability as well. Unfortunately, I’m not very skilled at the piano, so I just play a few scales in the following video. The piano keyboard in the video is an open source tool called VMPK (Virtual MIDI Piano Keyboard), which I did not write, but am simply using.

The video shows selecting a MIDI input source for the emulator and using the Channel Editor feature to show the patches assigned to MIDI channels. Then, a few scales are played using a few different patches.

MIDI Output

The ApOPL3xy also has a MIDI output port, which sends MIDI messages from the MIDI player to an external device. It also optionally echoes messages received from the MIDI input port, but that echoing is not demonstrated here. What is demonstrated is using VMPK as an external device to visualize the notes being played. In this video, VMPK is configured to display each MIDI channel with a different color. The MIDI file being played is the battle music from Final Fantasy VII.

Implementation Details

The ApOPL3xy firmware interacts with several pieces of hardware, all of which need to be emulated to be able to run the firmware purely in software.

Microcontroller

The hardware ApOPL3xy has an ATmega1284p microcontroller to run the firmware. Because the main goal of the emulator is to support debugging, this microcontroller is not emulated. Instead of running the compiled AVR firmware on an emulated ATmega1284p, the firmware is recompiled to native code for the computer running the emulator, with some conditionally-compiled hooks in a few places to tie in the other emulated hardware.

OPL3

This is probably the most important piece of hardware to emulate, and also the most daunting. Fortunately, the open source project DOSBox has code to emulate the OPL2 and OPL3 chips, so I adapted it for my use. This code exposes a function to set registers on the emulated OPL chip, which I used instead of the SPI interface I built for the hardware ApOPL3xy for that purpose. The DOSBox code also exposes a function to get output audio samples, which I used to populate the audio buffers (more on this later).

The OPL3 supports up to 4 channels of audio output, but sound cards incorporating the chip rarely if ever used more than 2. DOSBox didn’t bother implementing support for channels 3 and 4, so I modified it to add that support. It was actually easier than I expected. Since they had already gone to the trouble to implement 2 channels, most of the work involved increasing some array sizes and loop bounds from 2 to 4.

LCD Character Display

The standard 20×4 LCD character display used by the ApOPL3xy is based on a driver chip called the HD44780. I broke the emulation of this device up into two parts: the HD44780 emulation and the drawing of the LCD panel itself.

The HD44780 supports several commands which are sent by writing to its control and data registers. To emulate this chip, I wrote a C++ class to represent all of its internal registers, RAM, and ROM, and then implemented each command as a separate function that operates on the internal state. I then implemented a wrapper function which accepts commands or data, and dispatches to the correct underlying function(s). This is called by the emulated firmware instead of using SPI, similar to the OPL3.

For the HD44780 emulation, I also wrote a function to retrieve the output state, e.g., which pixels are on and which are off. This is used by the LCD panel emulation to actually display the pixels. I used the open-source, cross-platform GUI library wxWidgets to implement the emulator’s GUI, and the LCD panel is implemented as a custom widget that does its own drawing with the graphics routines provided by the library.

Audio Output

To implement audio output for the emulator, I used the open-source, cross-platform library PortAudio. This library abstracts all the platform-specific audio APIs and presents a unified interface to interact with the audio system. With it, the emulator is able to select audio output devices, sample rate, latency, and number of channels. (These options are exposed to the user via the Audio Settings dialog window.) To play the synthesized audio, the emulator extracts audio samples from the emulated OPL3 and sends them to the host audio system using PortAudio’s interface.

VU Meters and Gain Controls

Each output channel on the physical ApOPL3xy has a VU meter (well, not a true VU meter, just a linear amplitude display) and a gain control. Emulating this in wxWidgets was pretty straightforward. These are in separate tool windows that the user can display and dismiss.

The gain controls are slider widgets. To implement gain, the emulator reads the values from the sliders and uses them to adjust the audio samples before sending them to PortAudio for playback.

The VU meters are custom widgets that draw themselves based on a given amplitude. The emulator determines the current amplitude of each channel from the audio samples before sending them to PortAudio, and sends the amplitude values to their respective VU meter for display.

MIDI Input and Output

To emulate the MIDI input and output ports, I used the open-source, cross-platform library RtMidi. This library abstracts the host computer’s MIDI system, and enables enumeration of, reading from, and writing to system MIDI ports. The MIDI In and MIDI Out dialogs allow the user to select which, if any, MIDI ports to read from and write to.

If a MIDI input port is selected, MIDI events are read from it and injected into the firmware’s MIDI queue, instead of reading from the microcontroller’s serial interface. Similarly, instead of writing output MIDI events to the microcontroller’s serial interface, the emulator writes them to the host MIDI port selected for output.

MicroSD Card

The hardware ApOPL3xy includes a MicroSD card reader, which is read by the firmware using the open-source SdFat library. This library abstracts both the SPI messaging needed to interact with the card itself, as well as handling the FAT filesystem that keeps track of which files are where in the card’s storage.

To emulate this, I wrote a mock implementation of the subset of SdFat’s interface that the firmware uses, but instead of accessing an SD card, it uses the standard C++ std::filesystem library to access the host’s filesystem through the host OS. The user can “insert” a virtual SD card by using the Mount SD Card dialog window to select the directory to serve as the root directory of the virtual card. The user can “eject” a virtual SD card by using the Unmount SD Card menu option.

SRAM and EEPROM

The ATmega1284p microcontroller has 16 kiB of RAM and 4 kiB of EEPROM storage built in. This isn’t enough for everything I wanted the ApOPL3xy to be able to do, so I added SPI SRAM and EEPROM chips (128 kiB each) to the design. The SRAM on the microcontroller itself doesn’t need to be emulated; the emulator just uses the host’s system RAM for that. The microcontroller’s EEPROM isn’t used by the firmware at all, so that doesn’t need to be emulated either. The add-on chips, however, do need to be emulated.

The SRAM chip was very simple to emulate. I simply allocated an array of the right size and provided functions to read from and write to it. The emulator uses these functions instead of sending SPI commands like the firmware does on physical hardware. The EEPROM was very similar, except that a file is used instead an in-memory array, since EEPROMs are nonvolatile.

If the emulator finds no EEPROM file in the expected location on startup, it generates a default one and uses that. This has turned out to be useful for setting up a hardware ApOPL3xy. Since the format of the EEPROM is identical between the emulator and the hardware, one can copy the EEPROM file from the emulator to a MicroSD card and load it onto the hardware.

Input Controls

The ApOPL3xy has 10 buttons and 2 rotary encoders (which can also act as buttons) which are used to navigate menus and issue commands. The hardware version of these are connected to a shift register, which the firmware reads via SPI, interprets, debounces, and adds to an input event queue. The emulator instead provides GUI buttons for these, and pressing a button injects an input event directly into the queue. These buttons are also mapped to (hopefully) intuitive keyboard shortcuts.

Miscellaneous

There were a few other things that needed to be replaced or mocked to be able to compile and run the firmware in an emulated context.

• The microcontroller framework used by the ApOPL3xy’s firmware provides functions for timing that use the microcontroller’s on-chip timers. These were replaced with versions that use the standard C++ std::chrono library instead.
• The AVR architecture used by the ATmega1284p requires the use of special functions to read data out of program code (e.g. constant string data) instead of SRAM. These were replaced with simple pass-through functions, since the host machine for the emulator doesn’t have this restriction.
• Various utility functions like min() and max() provided by the framework needed to be reimplemented.

Download Links

Emulator

If you’re interested in trying out the ApOPL3xy emulator, you can download pre-compiled binaries here:

• ApOPL3xy Emulator v1.0.1 (Windows)
• ApOPL3xy Emulator v1.0.1 (MacOS)
• ApOPL3xy Emulator v1.0.1 (Ubuntu 24.04)
• ApOPL3xy Emulator v1.0.1 (Fedora 42)

Note that on Windows and MacOS, the fact that this binary isn’t signed may cause the operating system to complain. To get past this on MacOS, see https://support.apple.com/guide/mac-help/open-a-mac-app-from-an-unknown-developer-mh40616/mac. Microsoft doesn’t seem to have a good page on this, but here’s a potentially helpful google search.

Music Files

Here are some music files that you can play using the ApOPL3xy:

• VGM Files
• MIDI Files

Source Code

The combined source code for the emulator and firmware is available below. It’s released under the terms of the MIT License, except for the OPL emulation code from DOSBox, which is released under the LGPL 2.1 license. I’ll probably post this to github at some point, but the code is a bit messy at the moment, and I might like to clean it up first.

• ApOPL3xy Emulator and Firmware v1.0.1 (Source Code)

Hardware Design Files

If you’re interested and brave enough to want to build your own hardware ApOPL3xy, here are the files you’ll need. Note that the documentation is rather lacking at the moment, but I think someone determined enough could figure it out. If you try and have difficulties, let me know and I’ll be happy to help.

• ApOPL3xy PCB rev3 (Gerbers)
• ApOPL3xy PCB rev3 (Bill of Materials)
• ApOPL3xy PCB rev3 (Schematic)
• ApOPL3xy PCB rev3 (KiCad Design Files)
• ApOPL3xy Firmware v1.0.1 (HEX File)
• ApOPL3xy EEPROM Image v1.0.1 (BIN File)
• ApOPL3xy User Manual v1.0.1 (PDF File)

Closing Thoughts

I’m quite pleased with how this project turned out. What began as an attempt to make debugging easier grew into something kinda cool on its own. It also enables me to share the ApOPL3xy with more people than I would have otherwise. If you do try it out, I’d love to hear what you think. I have my own ideas of what could be improved, but there’s no substitute for real user feedback.

MIDI Player

One of the things I’ve added to the firmware recently is the ability to play MIDI files from the SD card. Here are a few videos of this in action. These videos are a little dark. It was hard to find a balance between blowing out the display and making the videos way too dark, so I did the best I could. I mapped the MIDI channels to somewhat arbitrary output audio channels to make the VU meters a little more interesting.

PCB Photos

Here are some high(ish) resolution photos of the assembled Rev 3 board, for those who are into that sort of thing. Also included is a photo of the bodges necessary on the Rev 2 board. To view the images at full resolution, right-click (or long-press) the image and select “Open Image in New Tab ” or similar.

Future Work

Now that the ApOPL3xy has reached this milestone, I’ll probably leave it alone for a while. But, such things are never truly finished. I have several ideas for user interface improvements, such as patch and bank copying, M3U playlist support, and most interesting to me, a drum machine mode (suggested by a co-worker). I’m sure I’ll come back to this eventually to make these and other improvements, but it sure feels good to be able to call it done (for now)!

Updated Music

Off and on over the past several months, I’ve been working on a 3D voxel-art version of Damon for modern computers. It’s almost finished, but it has been for a while. One day I’ll finish it and make a post about it. It just needs a menu screen for configuring the controls. But, that’s not what this post is about.

I mention the 3D game because as part of that project, I wrote an updated version of the game’s music. The original music, being for a 1980’s 8-bit computer, was a bit simple and repetitive. I thought I might be able to jazz it up a bit, so I gave it a shot in Ableton Live. I’m pretty pleased with the result.

Back-Porting

But, as I said, this post isn’t about the 3D game; it’s about the version for the Commander X16. I was looking through the VERA documentation recently, and I learned that it supports PCM audio. (VERA stands for Versatile Embedded Retro Adapter, and it’s the primary graphics and sound chip used by the Commander X16. PCM stands for Pulse-Code Modulation, which is a way of encoding digital audio.) I thought it would be fun to see if I could get the Commander X16 version of the game to play the music from the 3D game. There were a few challenges to overcome, but I was ultimately able to get it working.

Resolution Reduction

PCM audio is made up of a sequence of “samples”, each of which represents the value of the audio waveform at an instant of time. For example, CD-quality audio and the Damon 3D music both use two channels of 16-bit samples (-32,768 to 32,767) at 44,100 Hz (samples per second). Doing the math, this ends up taking 176,400 bytes per second, or just over 10.5 MB for a minute of audio. While this is trivial for modern computers, it’s way too much for something like the Commander X16.

The VERA PCM system supports a sample rate of up to 48,828.125 Hz (25 MHz / 512), and the rate can be configured by specifying a value between 0 and 128, inclusive, scaling linearly between 0 Hz (no playback) and the maximum rate. After playing around a bit, I settled on the value 21, which gives a rate of approximately 8,010.86 Hz. I used Audacity to resample the audio from 44,100 Hz to 8,011 Hz, which is close enough.

With this sample rate reduction, by mixing the stereo down to mono, and by using 8-bit samples instead of 16-bit, 1 second of audio can be brought from 176,400 bytes/second down to 8,011 bytes/second, a roughly 95.5% reduction in size! This obviously reduces the audio quality, but surprisingly, it sounds pretty okay despite the severe reduction in resolution.

One interesting thing to note is that the WAV file format treats 8-bit samples as unsigned offset binary, but VERA treats 8-bit samples as signed two’s complement. To handle this difference, it was necessary to write a python script to parse and convert the PCM data from the WAV files.

Tempo Adjustment

The new version of the music was rendered at 120 bpm. The original music was a bit faster, at 148 bpm. Since I wanted the player to be able to switch between the two versions of the music, I needed the two versions’ tempos to match up. With all the extra percussion in the new version, I felt like it sounded right at 120 bpm, so I adjusted the game code to generate the older music at that tempo as well.

I think the older music is a little better at the faster tempo, but that would probably be too fast for the new music, and it seems more important to have the two versions sync up.

Clip Sequences

To reduce duplication of redundant sections of audio, and to have more control over what parts of the music get played when, I broke the audio up into 1-measure clips, which at 120 bpm are 2 seconds each. For example, the music includes several measures of the bass line playing with no melody over it. There’s no need to store more than one of those. Similarly, other musical phrases are repeated in the song, and the game only stores a single copy of each distinct phrase.

There are a few other special-purpose audio clips, such as the ominous drone as the letters slide in on the title screen, and the level start and level complete music. Each of these is stored in its own file as well.

The audio clip files are stored in the same filesystem directory as the main program, and the first thing the game does is load each of them from disk into high memory. A stock X16 has 512 kiB of memory, and this is expandable to 2 MiB. All the clips together, after the resolution reduction, use about 220 kiB, so there was no problem fitting them all in. High memory is banked in the Commander X16, each bank appearing at $A000–$C000 when it’s selected. This didn’t present significant difficulty, as the PCM data is mostly read sequentially.

To handle the sequencing of audio clips I wrote a small interpreter with instructions to do things like “play this clip” or “jump to this other part of the sequence”, which made it much simpler to manage the clips and play the right one at the right time. I discussed the interpreter technique briefly in my previous post. If you’d like more information about this, you’re welcome to check out the source code on github, or ping me directly.

Gameplay Video

Here’s a short recording I made with the X16 emulator of the game using the new music. To compare this to the previous music, check out the gameplay video in my previous post.

Conclusion

I’m quite pleased with how this experiment turned out. I think it adds a bit more fun to the game. If you’d like to try it yourself, you can find it at natebarney.com/damon or on the Commander X16 forums. The source code is available at github.com/natebarney/damon-the-rocket-jockey.

About two months ago, I received in the mail my very own Commander X16 computer. This is a modern 8-bit computer envisioned and produced by David Murray (a.k.a The 8-Bit Guy) along with a thriving community. It’s inspired by Commodore computers from the 1980’s, primarily the Commodore 64. I hooked it up and started playing with it, and immediately fell in love with it. It’s so Commodore-y. But it runs at 8 MHz instead of 1 MHz, has VGA output, and has a whole bunch of other modern improvements. It came with several games and other pieces of software, and I enjoyed messing around with those for a while.

What I really wanted to do, however, was write something for the machine. But what to write? A game would be the most fun to work on, but game design is not my strong suit, nor are graphics or sound design. After giving it some thought, I decided to make a clone of an existing game, relying on the game, graphics, and sound design from that game. I chose to clone Nomad: The Space Pirate, a Commodore 64 game I played a lot as a kid.

I thought this was a good game to clone for several reasons. First, It’s a fun game (at least I think so). Second, it’s relatively simple; there’s a single static screen per level, and only a few moving objects, so no scrolling or crazy sprite multiplexing would be required. Third, it’s a pretty obscure game. I haven’t met anyone outside my immediate family that’s ever heard of it. This obscurity meant that I would be unlikely to be beaten to the punch while my version was still in development. If I’d chosen something like Pac-Man or Tetris, on the other hand, my game would probably only be one of many similar games available on the platform.

I also needed a name for the project. I couldn’t simply call it Nomad: The Space Pirate; that would be at best confusing, and at worst copyright infringement. I noticed that Nomad spelled backwards was Damon. That’s a good start, but if Nomad is a Space Pirate, what should Damon be? I decided that he should be a Rocket Jockey, which sounds cool and is fun to say. And, he does ride a rocket around, so it’s not inaccurate. Thus, Damon: The Rocket Jockey was conceived.

The Game

The goal of the game is to fly your ship around a grid, picking up pellets. Every level has one or more enemies, which the player must avoid or shoot, as crashing into them causes the player to lose a ship. Shooting an enemy destroys it, but it immediately respawns in a corner of the screen unless all the pellets have already been collected.

Once all pellets are collected, and all enemies destroyed, the player advances to the next level. Collecting pellets and destroying enemies award points, and a bonus ship is awarded every 10,000 points. Each level, another enemy is added (up to a maximum of 5), and in every level after the first, barrier blocks are placed in different locations around the play area. Neither the player nor enemies can fly through the barriers, but the player can shoot through them.

If you’d like to play the game, I’ve made it available at natebarney.com/damon. This page uses the Commander X16 web emulator to run the game right in your web browser. It doesn’t really work on smart phones or tablets however, so you’ll need to access it from a desktop or laptop computer. (For Science, I tried it on the Playstation 4 web browser. Unsurprisingly, it didn’t even load.) There are also links there to download the game if you’d prefer to play it using the standalone Commander X16 Emulator or even a real Commander X16.

If I’m having a really good game, I can sometimes make it to level 5, with my top score being something like 13,000 points. I’ve never made it to level 6, usually running out of ships at level 3 or 4, and my average score is something like 7,500 points. Here’s a video of me having a decent game, although I struggled a bit during level 3. It was captured directly from the VGA output of my physical Commander X16.

Development

The rest of this post will cover the development process for the game and discuss how or why I did things the way I did them. I probably won’t dive much into the actual code, but if you’d like to have a look at that, it’s available on GitHub.

Goals

My primary goal for this project was authenticity. I wanted my version to look, sound, and feel like the original. I think I largely succeeded, but there are, of course, some minor differences.

Another goal, perhaps related to the first, was to write the game entirely in assembly language. The original was almost certainly written this way, and I wanted to do the same. I did allow myself the luxury of using a modern cross-assembler (ca65) and linker (ld65), but I think that’s a minor concession. If I try my hand at writing another X16 game, I will probably try writing it in C. The cc65 suite includes a C compiler, and I haven’t done C for the 6502 before, so that should be interesting. But for this project, I stuck to assembly language.

My third goal was to write everything myself, from scratch. The author of the original game wrote what he wrote, and it’s copyrighted. Plus, it’s more fun to figure everything out myself. So, I didn’t look at the disassembled code for the original. I did use the machine language monitor in the VICE emulator to locate the memory address where the number of the player’s remaining ships was stored, so I could cheat by giving myself extra lives to see what the higher levels looked like. (This game gets hard pretty fast!) But, I didn’t look at the code.

Finally, I set myself a challenge goal of making my game smaller (in bytes) than the original. If I met the other goals but didn’t meet this one, I’d still consider the project a success, but it’s fun to see how tight one can make one’s code. As it turns out, the original was approximately 15K, and when finished, my version was just over 11K, so I count that as a win.

Graphics

The graphics for this game are primarily tile-based, with stationary graphical elements and text being composed of a 40×30 grid of 8×8 pixel tiles with 1 bpp (bit per pixel), giving a total screen resolution of 320×240. This is actually one of the minor differences I mentioned before. The Commodore 64 has a screen resolution of 320×200 pixels, or 40×25 tiles, but both systems use a 4:3 aspect ratio. This makes the C64 have a more vertically stretched appearance, and is the reason my clone has black bars at the top and bottom of the screen.

The elements that move (e.g. ships and bullets) are made using sprites instead of tiles. The Commander X16’s video chip, the VERA, supports up to 128 hardware sprites, of varying sizes and color depths, which it can overlay onto the stationary tiles at more-or-less arbitrary positions.

One of the first steps I took when developing this game was to grab a bunch of screenshots of the original game using VICE, and hand-copy out all of the graphics onto graph paper. I then used these notes to enter the tile and sprite data into assembly source code files, to be used by the rest of the game.

Tiles

The border around the screen, the pellets, and the blocks in the play area are all static graphical elements drawn using tiles. The blocks, and somewhat surprisingly, the pellets, are actually 16×16 pixels each, so they’re made of 2×2 arrangements of tiles.

The level 1 (green) and level 5 (light red) blocks are made of four different tiles each, but the blocks for levels 2-4 (brown, medium grey, and orange) and the barrier (light grey) blocks simply repeat the same tile 4 times. Levels 6-10 use the same block graphics as 1-5, but with different colors and barrier block layouts. Levels 11-99 are exactly the same as level 10. The border (purple) is a single tile thick. Interestingly, some of the individual pellet tiles do double duty as single quotes and periods on the title screen.

Text in the game is also displayed using tiles. I was able to capture almost all of these tiles from the original game, since most of them were used in various places, especially the title screen. I did have to guess at a few letters and symbols, J, W, X, Z, and :, that weren’t used anywhere in the original.

Sprites

A sprite is an image that the video chip can overlay on top of the tiles, at arbitrary positions not necessarily constrained to the tile grid. Each sprite has an independent position attribute, and simply updating the position moves the whole sprite image. This allows movement to be implemented in an efficient way.

Sprites on the Commodore 64 are 24×21 pixels, 1 bit-per-pixel (There is a 2 bpp mode, but Nomad didn’t use it.) The VERA chip supports sprites that are 8, 16, 32, or 64 pixels wide, and 8, 16, 32, or 64 pixels high. These dimensions may be mixed, so one could define a 64×8 sprite if one wanted to. Each sprite on the VERA may also be either 4 or 8 bpp.

There’s quite a bit of mismatch between the sprite capabilities of the two systems. Fortunately, both the player and enemy ship sprites fit nicely within 16×16, and the bullet sprites similarly fit within 8×8. That works well, but there’s still a color depth mismatch. Since VERA doesn’t support 1 bpp sprites, I wrote a routine to read 1 bpp sprite definitions out of system RAM, and pad them out to 4 bpp when copying them to video RAM (VRAM). That way, I didn’t need to store a bunch of unused color depth data in the program itself.

VERA has the ability to flip each individual sprite either horizontally, or vertically, or both (or neither). This was useful, as each ship needs to be able to face 4 different directions. By making use of this feature, I only needed to store a horizontal definition and a vertical definition per ship, instead of all four.

Animation

Sprites are also useful for animation. Each sprite entry in the video chip holds a pointer to the image data in VRAM that will be used to draw the sprite. It’s trivial to update the pointer and instantly change what the sprite looks like. The game uses this technique for player and enemy death animations, and for the large, animated block letters on the title screen.

When either an enemy or the player is destroyed, an explosion animation replaces the ship image. In the case of the player’s destruction, this is also followed by a winking skull and crossbones. Each of these animations consists of three separate frames. The explosion frames are played sequentially, and the winking skull frames are played sequentially and then in reverse.

Unfortunately, neither of these animations fit inside a 16×16 pixel sprite, instead making use of most of the C64’s 24×21 sprite resolution. To display these with the VERA, they need to be defined as 32×32 sprites. But, to save space, I defined them in the code using 24×21, and wrote a routine to pad them out to the required 32×32 when copying them to VRAM.

Title Screen

The block letters on the title screen both move and animate, so they’re also drawn using sprites. These sprites presented two new challenges, both related to features present on the C64 but not on the Commander X16, one much simpler than the other to resolve.

The first and simpler of the two challenges relates to sprite size. Recall that C64 sprites are 24×21 pixels. On the C64, but not the X16, sprites can be rendered as double-width, double-height, or both (or neither). This doesn’t add any extra detail to the sprite image. It merely doubles each pixel in the appropriate dimension(s) to increase the overall size.

The original game used this feature for the title screen block letters to make them double-width, for a total size of 48×21 pixels each. This means that, on the X16, the sprites need to be horizontally doubled and then padded out to 64×32. But, storing the doubled pixels directly in the game code would be wasteful. So, I wrote another sprite loading routine to double each pixel horizontally from 24×21 to 48×21 and pad the whole thing out to 64×32 when copying a sprite from system RAM to VRAM.

The second challenge was trickier. If you watched the gameplay video above, you might have noticed that the letters slide in from off-screen. The C64 allows sprites to be positioned partially or even completely off-screen, which makes achieving this effect relatively straightforward. However, the Commander X16 doesn’t support this. Setting a sprite’s X or Y position to 0 will put the sprite against the left or top edge of the screen, but the sprite will be fully visible. The coordinates are unsigned integers, so trying to set them to negative values results in large positive values instead, and the sprite isn’t displayed at all.

The key to solving this puzzle is that, on the X16, you don’t always have to set the sprite image pointer at the beginning of the sprite image data. So, I padded the sprite image data with extra blank lines at the bottom, and as a new letter is just starting to emerge from off-screen, I position the sprite at the very top of the screen. I then set the sprite image pointer to the first blank line past the actual sprite image data, so all that shows is a blank sprite. Then, each frame, I move the sprite image pointer one line up, to give the illusion of the sprite coming in from off-screen. As soon as the sprite image pointer is at the top of the sprite image data, I switch from moving the sprite image pointer up to moving the sprite position down. This gives a seamless transition as the letter continues to move down the screen.

Sound and Music

The sound effects in the game are pretty simple. There are only three sound effects: the player’s bullet sound, the ship destruction crash sound, and the bonus ship sound. I implemented all of these using the Programmable Sound Generator (PSG) that’s built into the VERA. To determine the parameters of the sounds, I recorded them from the original running in VICE, and loaded them into Audacity, an open-source audio editor. This allowed me to visually see the waveforms used and measure durations, and the fourier analysis tool it provides enabled me to determine roughly the frequencies to use.

The bullet sound uses a triangle waveform and does a linear sweep from about 880 Hz down to about 546 Hz over roughly half a second. It does stop whenever the bullet disappears, such as when it hits a wall or an enemy, so it might not make it to the end frequency. The ship destruction sound uses a noise waveform at about 4,400 Hz, and the base frequency doesn’t change, but the volume drops off linearly from full to zero over about half a second. The bonus ship sound uses a sawtooth waveform, with a constant frequency of about 1,134 Hz at full volume for about a sixth of a second.

The music is also pretty simple. The title/background music is the same 8 notes repeated endlessly, although the in-game tempo is slightly faster than the title screen tempo. It reminds me of the bass line from the Peter Gunn Theme. It is pretty repetitive, and in fact, a member of the Commander X16 forums asked for a button to turn it off! I kinda like it myself, but maybe that’s just nostalgia talking.

The other two pieces of music in the game are the level start and level complete jingles. The level start music is simply a rapid chromatic scale from A♯3 to G4. The level complete music is the most complex of all of them, and my friend Matt pointed out that it’s actually a sped-up, minor version of the opening measures of Pictures at an Exhibition: Promenade by Modest Mussorgsky.

For all of the music, I used the Yamaha YM2151 (OPM) FM synthesis chip built into the Commander X16. The X16 comes with a full set of synthesizer patches for this chip, burned into the ROM, and I played with some of those for a bit, but none of them sounded quite right. I ended up making my own patches by tediously tweaking the parameters until I got a sound that I felt was close enough to the original. The OPM chip’s parameters felt unusual to me, since my experience with FM is primarily with the OPL line of chips. For example, the envelope generator on the OPM has two separate decay phases. But, after playing around for a while, I think I’ve figured it out well enough.

The patch for the title/background music uses FM to approximate a sawtooth waveform, and the envelope has a sharp attack and decay, but a slow release, sort of like a plucked string. The patch for the level start and level complete jingles uses a similar sawtooth waveform, but has a slightly softer attack and decay.

Enemy AI

I fretted about the enemy AI for weeks before implementing it. After all, it’s what really makes the game challenging and fun to play. It seemed like a very complex part of the game to write, so I kept putting it off and worked on other parts, letting the problem simmer in the back of my mind. Soon enough, the time came when I couldn’t make any more progress without addressing this problem. Fortunately, a thought had occurred to me that might make it tractable: Markov chains.

A Markov chain is a process that makes decisions based only on its current state. Any previous history is irrelevant to what happens next. Instead of following some complicated overarching strategy, the enemies could simply make decisions in the moment, with whatever information they have at the time. I realized that the only time an enemy needs to make a decision is when it’s at an intersection. Any other time, it just keeps moving forward. So I watched the enemies move in the original game with this viewpoint in mind, to try to determine what rules it used. I came away with a set of rules that seems to be pretty close.

These are the rules I implemented for an enemy in its normal state:

1. If the enemy is at a wall, start moving in the reverse direction.
2. If the enemy has line-of-sight to the player, to the left, right, or straight ahead, move toward the player.
3. If the enemy is facing a barrier block, turn left or right with equal probability.
4. Otherwise, decide whether to turn with 50% probability, and if so, turn left or right with equal probability.

That’s all there is to it. If there are multiple enemies, they don’t coordinate. Each follows the same set of rules independently. It’s surprising how complex and apparently organized the resulting behavior can seem. However, this is for enemies in their normal state. Respawning enemies follow a different set of rules:

1. Select the corner of the screen furthest from the player and respawn there.
2. Start moving vertically away from the corner.
3. If the player is visible at an intersection, turn in there and enter normal state.
4. Otherwise, at each intersection but the last, turn in and enter normal state with 25% probability.
5. At the last intersection, always turn in and enter normal state.

However, there is one more aspect of enemy behavior that I have yet to cover. Some readers may have noticed that in the ship and bullet sprite figure above, there appears to be a bullet for the enemy. That’s exactly what that is. The enemies can shoot back.

It only starts happening at level 8, so it’s kind of ridiculous to even have bothered implementing it, since I don’t think anyone will ever reach level 8. But I’ve been wrong before, and in any case, the original game does it, so I wanted my clone to do it too. I only discovered it myself when cheating to see the higher level layouts.

The enemies only have one bullet to share among themselves. The Commodore 64 can only display 8 sprites at a time, unless the game does something fancy like sprite multiplexing, which Nomad apparently doesn’t do. So, we have the player, the player’s bullet, and 5 enemies, which comes to 7 sprites, leaving one left for the enemy bullet. I wonder if the enemies max out at 5 because the programmer wanted to save a sprite for the enemy bullet, or if 5 enemies was selected for other reasons, and since there was one sprite left over, he decided to use it for this. I guess there’s really no way to know for sure.

The first time I implemented enemy fire, the enemies were deadly snipers, immediately shooting and killing the player as soon as they had a shot. In order to tone this down a bit, I added a couple of constraints. The first was a reload time. At the beginning of a round, and after each time an enemy fires, the enemies have to wait for their bullet to reload before they can fire again. I set this to 5 seconds, because that seemed to approximate the rate of enemy fire in the original game. The second constraint was reaction time. Once an enemy sees the player and decides to fire, it won’t actually fire until 400 ms later.

Here are the resulting enemy fire rules, processed every frame in which the enemy isn’t busy making other decisions:

1. If the level is less than 8, don’t fire.
2. If the bullet is still reloading, don’t fire.
3. If the bullet is already in flight, don’t fire.
4. If the enemy is pointing at the player, start a 400 ms countdown and fire when it elapses, if possible.

These rules seem to result in behavior that loosely approximates that of the original game. Since it will likely never happen in practice, I think that’s close enough.

Collision Detection

Sprite collision occurs when two sprites overlap on the screen. It’s important to be able to detect when this happens, and between which sprites, to know when an enemy or player ship should be destroyed. The VERA chip used by the Commander X16 supports hardware collision detection, but the way it works can be a little confusing at first.

Hardware Collision Detection

Each sprite has a 4-bit collision mask, which can be thought of as defining 4 independent collision groups, one per bit. Each bit in the sprite collision mask is set to 1 for groups the sprite is a member of. Other bits are set to 0. The VERA keeps track of a 4-bit overall collision result, which it sets to all 0’s at the start of each frame. As the VERA renders each pixel, if it draws a sprite on top of another sprite, it checks their collision masks by bitwise-ANDing them together. If the result of the AND is non-zero (i.e. the two sprites share at least one collision group), the previous overall collision result is bitwise-ORed with the result of the AND to give the updated overall collision result. At the end of the frame, the overall collision result contains a 1 bit for each collision group that experienced a collision during that frame.

The program/game can register to receive a CPU interrupt for sprite collisions by specifying a 4-bit value (let’s call it the collision interrupt mask) indicating which collision groups it’s interested in. If, at the end of the frame, the overall collision result is non-zero, it is bitwise-ANDed with the collision interrupt mask. The result of this AND isn’t stored anywhere, but if it’s non-zero, the VERA generates a CPU interrupt and reports the overall collision result as part of its interrupt status register.

When the CPU receives an interrupt, it stops what it’s doing and jumps to a special routine called the Interrupt Service Routine (ISR), commonly known as an interrupt handler. The handler checks various hardware registers to see what generated the interrupt, and performs any actions that are needed to handle that interrupt. When it’s done, the interrupt handler returns the CPU to what it was doing before the interrupt occurred.

It’s good practice not to spend too much time in an interrupt handler. The interrupt handler for this game, when it receives a sprite collision interrupt, simply stores the overall collision result from the interrupt status register into a location in memory, to be processed as part of the normal game loop.

This game uses 3 different collision groups, one for the player’s ship, one for the player’s bullet, and one for the enemies’ bullet. Here’s a table of the collision group memberships:

This works really well for a single enemy. The VERA checks for collisions, and the set of groups with collisions tells the game exactly what needs to happen. If the player ship group or enemy bullet group has a collision, the player ship is destroyed. If the player bullet group has a collision, the enemy ship is destroyed. However, this runs into difficulties when multiple enemies are introduced. To resolve them, some software collision detection is also needed.

Software Collision Detection

When there is more than one enemy, the collision groups alone don’t provide enough information, so the game needs to do some additional checks to determine what needs to happen next. For example, if the player bullet group has a collision, that doesn’t specify which enemy was hit.

To resolve this issue, the bullets and ships are assigned “hit boxes,” which are axis-aligned rectangles relative to the sprite’s coordinate space. (Axis-aligned means each side of the rectangle is parallel to either the X- or the Y-axis.) Hit boxes are entirely a software concept; the VERA chip has no notion of them. When the VERA informs the game of a collision in the player bullet group, for example, the game does an intersection test between the bullet’s hit box and each enemy’s hit box. If it finds an intersection, that’s the enemy that should be destroyed.

The player bullet group isn’t the only group that needs these extra checks, however. Every enemy is also a member of the player ship collision group. This means they can collide with the player ship, as expected. Some readers might be wondering why it would matter which enemy the player collided with. It, of course, doesn’t matter, as far as that goes. But since enemies are all in the same collision group, when they collide with each other, that registers a collision in that group. Without extra checks to determine whether the player was involved, any time two or more enemies overlapped, the player’s ship would spontaneously explode. I actually tried that experiment, For Science, as I was implementing multiple enemies. It was pretty funny to watch.

The game is able to rely solely on hardware detection for collisions between the player ship and enemy bullet, however, since there’s only one of each. It’s only when there are multiple sprites of a given type (e.g. enemies) that the software algorithm is required. Therefore, there’s no hit box defined for the enemy bullet.

There are a couple of reasons to use axis-aligned hit boxes. The first is that they only require four numbers to fully specify: top, left, bottom, and right. The second is that it makes intersection testing really simple. The trick to doing an intersection test between axis-aligned rectangles is to check all the ways that they might not intersect. If the left coordinate of one rectangle is greater than the right coordinate of the other, or vice-versa, they clearly don’t intersect. The same holds true in the other direction with the top and bottom coordinates. If none of these four checks returns true, then the rectangles do intersect. Neat, huh?

The significant advantage to using hit boxes for software collision detection is that it’s much faster than checking each sprite against each other sprite, pixel by pixel. This could be done, but it would be very computationally expensive (and harder to write as well). The obvious disadvantage is that it’s less precise than the pixel-perfect hardware collision detection. To deal with this, I shrank the player ship’s hit box a little. Otherwise, there could be spurious collisions when two empty sprite corners “overlap.” Since the game is hard enough already, I erred on the side of forgiving.

If the VERA supported seven or more collision groups, this software collision detection would be unnecessary (for this game). In that case, the one player ship collision group could be replaced with five new collision groups, one for each enemy ship, and the collision groups themselves would provide enough information about what to do. In fact, as I’m writing this, it occurs to me that if the enemies were partitioned into two groups, that could reduce the average number of checks the software collision detection would need to make. Fortunately, the game runs well enough without this optimization that I don’t feel the need to implement it.

Miscellaneous Techniques

In this section, I’ll describe several programming techniques that I found useful when developing this game.

Game Loop and Hierarchical Init / Update

The core of any game is the game loop. This is the bit of code that runs over and over again until the game exits, handling everything—input, graphics, sound, music, game state, etc. When the game first starts, it calls some initialization routines, often shortened to “init.” Once init is complete, the game enters the game loop, calling the update routine repeatedly.

I found it useful to establish multiple, hierarchical init and update routine pairs. There are of course, the master init and update routines, at the top of the hierarchy. But, for example, the title screen has its own init and update routines, only called when the game is displaying the title screen. There are also pairs of routines for the player, the enemies, the bullets, the music, the sound effects, and several others. Each layer of the hierarchy knows when and whether to call its subordinate sets of routines. In this way, the game is able to manage multiple independent operations apparently simultaneously, without becoming a huge mess of spaghetti code.

V-Blank Interrupt

If the game ran its update loop as fast as it possibly could, this would cause multiple problems. Some iterations of the loop take less time than others, depending on what needs to be done each time. So, the game would speed up and slow down erratically. The Commander X16 itself can run at different speeds. It defaults to 8 MHz, but it can also run at 4 Mhz or 2 MHz, if the user desires. Changing this would alter the frame rate and behavior of the game as well. Finally, the graphics would be updated at effectively random times throughout the frame update, so at any one time, the screen would display part of one frame, and part of another. Possibly more than two, depending on how fast each loop iteration is.

To fix all of these issues, a common technique is to synchronize the game loop to the vertical blanking interval, or V-blank. A V-blank happens only once per frame, after the entire frame has been drawn. It also happens very regularly. The Commander X16 uses a frame rate of 60 Hz, so a V-blank happens pretty much exactly 60 times a second.

The VERA chip can be configured to cause a CPU interrupt on V-blank, so I enabled that, and set up my interrupt handler to set a flag variable to 1 when a V-blank interrupt occurs. The main game loop waits for that flag to become 1, performs a single update, sets the flag to 0, and waits for it to become 1 again. This allows the game to update at a smooth 60 frames-per-second, without any of the issues described above.

State Machines

Several of the entities in the game have complicated behavior. For example, an enemy can be moving, deciding whether to turn, exploding, or respawning. All of these behaviors could be implemented in a monolithic update routine, but that would make keeping track of all of the variables that might affect things a nightmare. Instead, entities with complicated behaviors were given a state machine.

With a state machine approach, the entity has a state variable, which roughly corresponds to “what it’s doing right now.” The behavior of the entity differs depending on what state it’s in, and some events can move the entity to a new state. For example, if the enemy is in the “moving” state, and it’s hit by the player’s bullet, it transitions to the “exploding” state, and begins playing the exploding animation. Once the animation is complete, it transitions to the “respawning” state, unless all the pellets in the level have been collected, in which case it transitions to the “retired” state instead.

State machines fit nicely into the hierarchical init/update scheme described above. Each state has an init routine, which is called when the entity transitions to that state, and an update routine, which defines the behavior of the entity while it’s in that state. Entities’ main update routines are often nothing more than a lookup of the state value and a dispatch to the appropriate state-specific routine.

Entities don’t have to be moving objects to benefit from state machines. For example, the game itself has states corresponding to displaying the title screen, displaying the “Get Ready” screen, playing the level start music, and running the main game. The title screen has states for sliding letters in and animating the letters. It’s an extremely useful concept for organizing game code.

Object-Oriented Design

In general, when developing software, I follow the motto “make it work, then make it pretty.” This means that, in the beginning, exploratory phase of development, code organization is given second priority to getting something working at all. Once the code works, then it’s time to refactor the code and make it nice and neat. Often, with larger projects, this cycle repeats more than once. At some point, I’ve usually gotten enough things working (but not pretty) that adding anything new is challenging. That’s when I know it’s time for a “make it pretty” phase. Once that’s done, I can add more features to the software.

During the development of this game, I went through several such cycles, and during one of the “make it pretty” phases, I noticed I had a lot of repeated code for entities that move around the screen (e.g. ships and bullets). I wanted to try to collapse this duplicated code down to one common set of routines, and even though I was working in assembly language, I found utility in the principles of object-oriented programming (OOP).

Of course, the OOP techniques I employed were rudimentary compared to the capabilities of languages like C++ or Java. Data encapsulation (e.g. private or protected), for example, is more of a high-level language feature, and in assembly language, there is no enforcement of such things. But, objects themselves can be done, or at least approximated, in assembly language just as well as in any high-level language.

Going back to the moving entity example, I identified all of the common variables used by these entities and grouped them together into a single data definition (struct). ca65’s .STRUCT directive was very useful for this. I was then able to declare multiple instances of that struct, one for each moving entity, and pass a pointer to that struct into the routines that implemented motion.

I even have one example of simple inheritance. The struct for enemies has all of the data members that a moving entity struct has, but it adds another after those to keep track of the reaction time delay for firing its gun. Because the enemy struct starts with all of the same data as the moving entity struct, I can pass a pointer to an enemy struct to a routine expecting a moving entity struct, and everything works.

Interpreters

A few things in the game needed to be “scripted,” or follow a predetermined sequence of steps. For example, the music follows a fixed sequence of notes, and animations follow a fixed sequence of frames. For each of these, to make changing the “scripts” easier, I implemented a rudimentary interpreter. Such an interpreter is given a list of commands, each with its parameters, stored in read-only memory (ROM).

The interpreter looks at the first command, executes it (i.e. performs the specified action), then goes to the next command in the list to execute that one. Some commands can change which instruction is next, to enable the creation of loops. This was useful for the title/background music as well as the title screen animations, both of which loop. For the animation interpreter, I implemented several commands, including sprite positioning, setting the sprite image, setting the sprite color, and a delay command. I was impressed by the utility of this approach, and I expect to use it again if/when I write another game.

Closing Thoughts

I am quite proud of what I achieved with this project. I feel I met all of my goals, learned a lot, and added something to the Commander X16 community. I think the game is fun to play too! I’ve spent several hours playing it already. As usual, if you have questions on any particular topic, feel I haven’t explained something well enough, have encountered bugs in the game, have a suggestion for improvement, or even have ideas for future projects or topics, I’d love to hear from you. Feel free to leave a comment below, or contact me directly if you have my contact info.

Microcontroller

The microcontroller that drives the system is the Microchip ATmega1284. (It used to be made by Atmel, before Microchip bought them.) The ATmega1284 is an 8-bit AVR microcontroller, similar to but more powerful than the ATmega328P at the heart of the Arduino Uno R3.

Hardware

Chips in the AVR line of microcontrollers have integrated flash memory for storing programs, SRAM for program variables and stack, and an EEPROM for non-volatile data storage. Most have hardware implementations of several communication protocols, such as SPI, I²C, and TTL Serial (basically RS-232 but with different voltage levels).

Whereas the ATmega328P has 32 kB of flash, 2 kB of SRAM, and 1 kB of EEPROM, the ATmega1284 has 128 kB of flash, 16 kB of SRAM, and 4 kB of EEPROM. I had initially designed the ApOPL3xy around the ATmega328P, but soon found the memory constraints too limiting, especially the 2 kB of SRAM. So, I upgraded to the ATmega1284. 128 kB of program flash and 16 kB of on-chip SRAM should be more than enough for the initial version, and leave plenty of room for future expansion.

The microcontroller in the ApOPL3xy runs at 5V and 20 MHz. A quartz crystal is used to generate the clock signal. 20 MHz is the maximum clock frequency for which the ATmega1284 is rated, and so far, it seems to be sufficient.

Programming

Arduino boards each have a USB port connected to a USB-to-serial bridge chip, which is used by special bootloader code on the microcontroller to receive new program code, so that all one needs to program an Arduino is a USB cable. When using a bare microcontroller as the ApOPL3xy does, the way to program it is ISP (In-circuit Serial Programming), sometimes called ICSP. This uses the SPI bus to upload code to the microcontroller (configured as the slave). However, to do this, one needs a separate device to act as the SPI master and upload the code to the microcontroller. I’m using the AVRISP-mkII programmer for this project. There are many others, but I have read that some of them have trouble with chips that have more than 64 kB of flash. I have, so far, not had any problems with the AVRISP-mkII, but note that the compiled firmware is still less than 64 kB in size, so that’s not entirely conclusive.

I’m using the PlatformIO IDE to edit, compile, and upload code to the microcontroller. This could be done with the official Arduino IDE as well (with the MightyCore board definitions added), and I was doing it that way for a while. However, the Arduino IDE is not very developer-friendly for large projects, and it makes simple things like having multiple source files much more difficult than they should be. PlatformIO, on the other hand, is much easier to use and I find myself fighting with it much less than with the Arduino IDE. PlatformIO is a Visual Studio Code extension, so you’ll need that installed, but it’s cross-platform, supports a large number of microcontrollers, and can use libraries written for the Arduino IDE. In theory, it even supports debugging over JTAG if the chip supports it. I haven’t tried that yet, but the ATmega1284 does claim to support it. PlatformIO is still a tad rough around the edges, but it’s so much better than the Arduino IDE experience that I highly recommend it.

Peripheral Devices

The ApOPL3xy has several peripheral devices attached to the microcontroller. “Peripheral” here means that the device is separate from the microcontroller chip, not necessarily that it’s detachable from the circuit board. Each of these peripherals communicates with the microcontroller using the SPI protocol. The one exception to this is the MIDI input port, which communicates using TTL Serial and is therefore connected to one of the microcontroller’s USART (i.e. hardware serial) interfaces.

The ApOPL3xy contains the following peripheral devices:

• OPL3 FM synthesis chip (Yamaha YMF262)
• 20×4 LCD character display module (with Hitachi HD44780-compatible controller)
• Input controls
  • 2 EC11 incremental rotary encoders
  • 10 tactile push buttons
• MicroSD card module
• 128 kB SRAM (Microchip 23LCV1024)
• 128 kB EEPROM (Microchip 25AA1024)
• MIDI 5-pin DIN input port
  • I’m thinking about adding a MIDI thru port as well
• Reset button

Reset Circuit

The reset button is connected to the reset pins of both the ATmega1284 and the OPL3. Since this signal is not being handled by a GPIO pin, software debouncing is not really an option. I also wanted the reset signal to start out active, so that the chips connected to it would be reset at power on. So, I designed a simple RC (resistor-capacitor) circuit, with its analog output converted to digital by means of two gates from a 74HC14 (six Schmitt-trigger inverters).

The reset signal for both chips is active low (as it is for most chips with a reset pin), so the way this circuit works is as follows. One lead of a 10 μF capacitor is connected to GND (0V), so the other lead starts out at 0V as well. The second lead is connected to V_CC (+5V) through a 10kΩ resistor and a 1kΩ resistor in series, so the capacitor gradually charges to +5V over time. After about 60 ms, the voltage at the second lead becomes high enough to trigger the first inverter, which goes low, and then the second inverter goes high, deactivating the reset signal.

When the reset button is pressed, it connects the second lead of the capacitor to ground through just the 1kΩ resistor, allowing it to discharge quickly, bringing the reset signal low (active). The signal stays low until the reset button is released, at which time, it takes about another 60 ms for the capacitor to charge up enough to bring the reset signal high again. If the button bounces when pressed, that high frequency oscillation is filtered out by the slow-charging capacitor, which is acting as a low-pass filter.

Here are a couple of captures from my oscilloscope illustrating the behavior of the circuit. In these images, the yellow trace measures the voltage across the capacitor, the pink trace measures the output of the first inverter, and the blue trace measures the output of the second inverter, which is the reset signal itself. The first image shows the behavior as the system is powered on. You can see the capacitor charging and how the schmitt-trigger inverters react to it.

The second image shows the behavior when the reset button is pressed. In this capture, you can see the capacitor quickly discharging as soon as the button is pressed, and remain discharged until the button is released, then start slowly charging again.

Custom SPI Interface

Three of the peripherals (MicroSD, SRAM, EEPROM) connected to the microcontroller natively communicate using SPI. The others do not, but in order to conserve GPIO pins, I built an SPI interface for them (except for the MIDI port) using shift registers. In a recent post, I described how this works in more detail. I could have used something a bit fancier, like the Microchip MCP23S17 SPI I/O expander. But they’re more expensive and more proprietary than standard 74HC shift registers, and the shift registers work just fine.

The SPI interface is built from one 74HC595 (serial-in / parallel-out shift register), two 74HC165‘s (parallel-in / serial-out shift registers), and one gate each from a 74HC14 and a 74HC125 (four tri-state buffers). I could have used a 74HC04 instead of a 74HC14, as I’m not using the Schmitt trigger functionality, but I already had a 74HC14 in the design for the reset button circuit, and I wasn’t using all of its gates. This setup provides eight output pins and sixteen input pins, and uses only three of the microcontroller’s pins. Two of these pins, SCK and MOSI, are shared among all SPI devices, so this really only uses one extra pin: SS.

Since the LCD character display and the OPL3 sound chip are controlled independently from one another, I was able to use the same eight output pins from the 74HC595 for both, and so they’re connected to each of these peripherals’ data busses. The sixteen input pins are connected to the two rotary encoders (three pins each) and the ten push buttons (one pin each).

OPL3 FM Synthesis Chip

If the ATmega1284 is the heart of the ApOPL3xy (and if you’ll forgive my briefly waxing poetic), then the Yamaha YMF262 (a.k.a. OPL3) is its soul. Well, perhaps brain and larynx would be a better metaphor; it just doesn’t have the same ring. But I digress…

The OPL3 is a sound synthesizer chip that implements frequency modulation (FM) synthesis. It was used in Creative Labs’ popular Sound Blaster 16 sound card released in 1992 for IBM-compatible PC’s, and many PC games of that era used it to produce their in-game music.

FM Synthesis

FM synthesis is a method of producing sound waves by combining simpler waves. The basic sound-producing unit in a synthesizer is the oscillator. An oscillator takes a few parameters: frequency/pitch, amplitude/volume, and waveform, and produces the sound wave with those characteristics. In the OPL line of synthesizer chips, and often with FM in general, oscillators are referred to as operators. The reason for this is the way FM synthesis uses oscillators.

Each voice, or independently controlled sound source, in FM is built from two or more oscillators. However, usually only one of these oscillators, called the carrier, actually produces sound. The other oscillators, called modulators for reasons which will soon be apparent, modulate the carrier by adjusting the carrier’s frequency up or down based on the amplitude of the waveform produced by the modulator. This can produce a wide variety of sounds, many of which approximate different musical instruments fairly well. With more than two oscillators, there may be multiple carriers sounding at once, and/or multiple modulators, modulating carriers or even other modulators. The reason FM oscillators are often called operators is that they operate on each other in this way.

The OPL3 contains thirty-six operators, which are paired together to provide eighteen independent two-operator voices. Up to six pairs of voices can be combined to create four-operator sounds, at the cost of a corresponding decrease in the number of simultaneous voices. If percussion mode is enabled, three two-operator melodic voices are exchanged for five percussion voices—one two-operator voice and four one-operator voices, for a total of twenty independent voices. (One-operator voices are not technically FM, since there’s no modulation happening, but they can be produced by the OPL3.)

Microcontroller Interface

The microcontroller interface to the OPL3 consists of a chip select pin (CS), a write enable pin (WR), a read enable pin (RD), two address pins (A0, A1), eight data pins (D0–D7), an interrupt request output pin (IRQ), and a reset pin (IC, or Initial Clear). To control the behavior of the synthesizer (e.g. play a note, adjust sound settings, etc.), the microcontroller uses these pins to set the value(s) of one or more registers within the chip. I won’t go into detail about the specific registers and their values, because the unofficial OPL3 Programmer’s Guide has already done an excellent job of that. They’re also described in the datasheet, but I find that does a rather poorer job.

To set a register, the microcontroller first needs to set the CS pin low (if it’s not already) to select the chip, the A0 pin low to indicate it’s selecting a register, the A1 pin either high or low to select one of the two banks of registers, and the D0–D7 pins to the index of the register to set. The WR pin is then pulsed low then high to write the register index. Next, A0 is set high to indicate the register’s value is being set, and D0–D7 are set to the new value for the register. The WR pin is once again pulsed low then high, this time to write the register value. Finally, if the microcontroller is done setting registers, it can set the CS pin high to deselect the chip.

The ApOPL3xy connects the OPL3’s pins as follows. CS is connected to GND to permanently select the chip. A0, A1, IC, and WR are connected to individual GPIO pins on the ATmega1284, configured as output pins. (A0 actually shares a GPIO pin with the LCD module’s RS pin.) D0–D7 are connected to the eight shared output pins from the custom SPI interface built from shift registers. This pin sharing works because the OPL3’s WR is only brought low (active) when any other device using the shared pins is inactive. When WR is high, the OPL3 doesn’t care what the values of the address and data pins are (except for setup and hold times, but those are so short that, even at 20 MHz, it’s hard to violate them; see the datasheet for more details on that).

The IRQ and RD pins are rarely used. The OPL3 can generate timer interrupt signals on the IRQ pin to let the microcontroller know when a certain amount of time has elapsed, and the RD pin is only able to be used to get information about these interrupts. The Sound Blaster 16 did not use this feature, nor does the ApOPL3xy. Therefore, the ApOPL3xy connects the RD pin to V_CC to permanently disable reads, and leaves the IRQ pin disconnected.

Digital-to-Analog Conversion

The OPL3 doesn’t generate sound directly. Rather, it produces digital representations of waveform amplitude (called samples) at a rate of 49,716 samples per second. The samples are represented as 16-bit offset binary numbers, where 0 represents the most negative value, and 65,535 represents the most positive value. These samples are sent serially to a Digital-to-Analog Converter (DAC) chip, the YAC512, which was designed as a companion chip to the YMF262.

The YAC512 takes the digital samples and converts them to an analog waveform which can be sent to an amplifier and then to a loudspeaker. Each YAC512 supports two audio channels, and the YMF262 can connect to two YAC512’s, giving four audio channels. Each of the OPL3’s voices can be configured to be output to any combination of the four audio channels.

LCD Character Display Module

This is a pretty standard component for a lot of homebrew projects. It’s a twenty-column by four-row character display module with an LED backlight and an integrated controller (Hitachi HD44780). It also comes in a sixteen-column by two-row variety, but I wanted the little bit of extra space afforded by the larger module. Ben Eater has an excellent video (to be honest, all of his videos are excellent) in which he connects a 16×2 version to his 65C02-based breadboard computer. The datasheet for the HD44780 is surprisingly good, and details all of the instructions that can be sent to the module.

The microcontroller interface consists of eight data pins (D0–D7) and three control pins: enable (E), register select (RS), and read/write (RW). There are five other pins, two for power, two for backlight power, and one to set the contrast of the display, for a total of sixteen pins, but only eleven need to be connected to the microcontroller.

To send instructions to the LCD module, a microcontroller first needs to set RW low, to signal a write, and RS low, to signal an instruction. Next, D0–D7 are set to the instruction to send, and finally E is pulsed high then low to send the command. Sending data (i.e.characters) follows the same process except that RS is set high to signal data rather than an instruction, and D0–D7 are set to the ASCII value of the character to send.

The LCD module also supports a four-bit mode, in which only D4–D7 need to be connected. This would be one way to conserve GPIO pins, but the ApOPL3xy doesn’t take this approach. This mode takes twice as long to send instructions and data. Because each instruction and character is still eight bits, each one takes two cycles to send. However, the main reason the ApOPL3xy uses the eight-bit mode is that it simplifies the code to do so, and the GPIO pressure is dealt with another way (see below).

Character data can be read back out of the LCD module, if desired, by setting RS and RW high, pulsing E high then low, and reading the value of D0–D7. There is also a “busy” flag which can be read by setting RS low, setting RW high, pulsing E high then low, and reading the value of D7. The busy flag indicates that the HD44780 is still processing the last instruction it received.

If an instruction is sent while the busy flag is high, the instruction will not execute, and the HD44780 will take longer to complete its current action, so this should be avoided. However, the datasheet lists the maximum duration for each instruction, so another way to avoid this situation is simply to wait long enough between instructions. This can be slightly slower, but the longest delay needed is about 1.5 ms, and most are less than 50 µs, so it’s not terrible, and it simplifies the connections if reading doesn’t need to be supported. Therefore, this is the approach the ApOPL3xy takes.

The ApOPL3xy connects the LCD module’s pins as follows. RS and E are connected to GPIO pins on the ATmega1284, configured as outputs. (RS shares a pin with the OPL3’s A0 pin.) D0–D7 are connected to the eight shared output pins from the custom SPI interface built from shift registers. As with the OPL3, this pin sharing works because the LCD’s E pin is only brought high when any other device using the shared pins is inactive. When E is low, the LCD doesn’t care what the values of RS, RW, and D0–D7 are.

Some readers might be wondering whether the ApOPL3xy uses the Arduino LiquidCrystal library to control the LCD module, and if not, why not. The ApOPL3xy doesn’t use this library because the library expects all of the LCD’s pins to be connected directly to the microcontroller’s GPIO pins, and that’s not how the module is connected in this case. To do so would have used more GPIO pins than I would have liked. The LCD control code in the ApOPL3xy’s firmware has an API modeled after LiquidCrystal, because it has a decent design, and because that might be more familiar to some developers.

Input Controls

The user interface for the ApOPL3xy consists of the LCD character display module previously discussed, and a number of input controls. Specifically, two EC11 rotary encoders, and ten momentary tactile push buttons. Each encoder has three output pins, and the ten push buttons have one each, for a total of sixteen. These outputs are connected to the sixteen inputs provided by the 74HC165’s in the custom SPI interface.

The shift registers in the SPI interface are part of the reason there are so many buttons. With six inputs needed for the two encoders, a single 74HC165 would only provide enough inputs for two more buttons. I felt that this wouldn’t be enough, so I added another 74HC165 with another eight inputs. Rather than let some of them go unused, I connected a button to each of them, for a total of ten. At present, the firmware only uses five of the buttons, but I’m sure I’ll be able to find uses for the others.

The buttons each have two pairs of pins, but the pins in each pair are shorted together, so there are effectively only two pins per button. I believe, but I’m not 100% certain, that the redundant pins are there to provide structural support when soldered to a circuit board. While the button is pressed, all four pins are shorted together.

EC11 rotary encoders are knobs that can be turned in arbitrarily many discrete steps in either direction, unlike a potentiometer, which turns continuously, but has limited extents. The encoders send a signal for each clockwise and counter-clockwise step, and they also serve as push buttons when pressed. If you’re curious to know how these work in more detail, check out my recent post on the topic.

These controls are all wired up in the ApOPL3xy to produce active-low signals. This means that each shift register input pin sees a low signal while, for example, the connected button is pressed, and a high signal otherwise. I don’t have a strong reason for making the controls’ outputs active-low rather than active-high. It would have worked just as well the other way.

The ApOPL3xy deals with the problem of contact bounce by applying a software debouncing algorithm. This algorithm acts as a state machine which only transitions states after a configured minimum time is spent in each state. In this case, the states are (effectively) idle, idle wait, active, and active wait. The wait states are the ones that require minimum durations before transitioning to its non-wait counterpart. If there’s interest, I could make another post going into detail about the various hardware and software debouncing solutions, and the pros and cons of each.

MicroSD Card Module

The ApOPL3xy includes a MicroSD card module to allow it to read VGM (and eventually MIDI) files for playback, as well as to load and save data to and from the EEPROM. I’m looking into the possibility of using a bootloader for the ATmega1284 that can load new firmware from the SD card, so an ISP programmer wouldn’t be required, but my investigation into that is not yet complete.

The MicroSD module I’m using was meant for Arduinos. It’s a small board with a slot for the card, a pin header to connect to the microcontroller, and a few extra components to shift voltage levels between the 5V the microcontroller uses and the 3.3V that the card uses. I may, in some future version of the ApOPL3xy, ditch the separate module and just incorporate a card slot and a level shifter chip directly. The card itself contains all the circuitry needed for the actual control and data interface, so it would be relatively straightforward. The module is convenient however, so that’s what I’m using for now.

SD (and MicroSD) cards natively use a four-bit parallel protocol, and that’s the way most modern devices (e.g. computers, smartphones, etc.) talk to them. Using this mode enables the highest transfer rates that the card can support. However, these cards also provide an SPI interface, which is how most smaller microcontrollers, including the ApOPL3xy, talk to them. This method is slower, but still fast enough for how the ApOPL3xy uses the card, it’s supported in hardware by the microcontroller, and it only uses up one additional GPIO pin for the SPI SS signal.

One issue I ran into is that SD cards are not particularly well-behaved when it comes to SPI. Specifically, the cards don’t seem to release the MISO line when its SS line is brought high, interfering with all the other devices on the SPI bus. At least this is the case with the module I’m using, but I believe this is a general phenomenon. To fix this, I used a 74HC125 tri-state buffer chip to disconnect the SD card’s MISO line when its SS line is high. In fact, I already had a 74HC125 with unused gates in the circuit, because I needed to do the same thing with the 74HC165 shift registers as well.

The SPI interface to the SD card provides raw read/write access to the bytes stored on the card. However, that’s not really sufficient to be able to read and write files on the card. At least, not if the card needs to be able to be read and written by other devices as well. Most storage devices, including SD cards, organize the large amount of data stored on them by using a filesystem. Filesystems keep track of file metadata like filenames and file size and provide controllers with a way to work with files instead of just raw data.

The topic of filesystems is vast, and well outside the scope of this article, but a common filesystem used on SD cards is called FAT32. FAT32 was developed by Microsoft for Windows 95, as an extension to their earlier filesystems FAT16 and FAT12, developed for MS-DOS. FAT32 has become a de facto standard for removable media, as its relative simplicity compared to other filesystems has enabled many different computer operating systems and embedded systems to implement support for it. This is slowly being superseded by Microsoft’s more modern exFAT filesystem, but support for this is still far from ubiquitous.

The ApOPL3xy expects its SD card to be formatted with the FAT32 filesystem, and rather than implementing a FAT32 filesystem driver from scratch, it makes use of the SdFat library. This library takes care of both the low-level SD card interface for handling raw data and the FAT32 filesystem interface. It’s been in development for many years, and at the time of this writing, is still in active development. It manages to pack a tremendous amount of capability into a surprisingly small amount of space, and does so very efficiently. It even has optional support for exFAT, but ApOPL3xy doesn’t enable that, at least not yet.

External SRAM and EEPROM

Even though the ATmega1284 has much larger memory capacities than the ATmega328P, it’s still not enough to store as many synthesizer patches (i.e. sound settings to emulate various instruments) as I want to be able to. Each patch for the OPL3 (at least as I have currently implemented it) is 23 bytes, plus a 24-byte name string, giving 47 bytes per patch. The General MIDI specification includes 128 melodic instruments, and 61 percussive instruments, for a total of 189 * 47 = 8,883 bytes, more than half of the available SRAM space, and more than double the available EEPROM space. Furthermore, I’d eventually like to be able to store and select from multiple banks of patches.

To deal with this, the ApOPL3xy includes an external (i.e. not part of the microcontroller) SRAM chip (Microchip 23LCV1024) and an external EEPROM chip (Microchip 25AA1024), each with 128 kB of space. These chips, like most of the other peripherals in the ApOPL3xy, communicate via the SPI protocol. The specific details of the command interface can be found in the datasheets for these chips, but the basic structure for both chips is: write a command byte (i.e. “read”, “write”), write a three-byte address, then read or write as many bytes as desired. Because these chips hold 128 kB of data, only seventeen bits are needed to encode an address, and the seven most significant bits of the three-byte address are ignored.

Writing to the EEPROM is slightly more complicated. Its storage is organized into 256-byte pages, and each write operation can only modify a single page. To write to multiple pages, multiple write operations must be performed. To write to a page, first the status register must be checked to ensure that the chip has completed its last operation. Then a “write enable” command byte must be sent, then the chip select line must be brought high and then low before the “write” command is sent, followed by the 3-byte address, followed by up to 256 bytes of data. Once the last byte of the page is written, the chip select is brought high, and the cycle is repeated for any additional pages.

MIDI Port

The ApOPL3xy uses the MIDI protocol to allow an instrument (like a keyboard) to control the FM synthesis chip. MIDI is a fairly straightforward protocol, and the electrical interface is reasonably simple to implement. The MIDI port itself is a female 5-pin DIN jack, and there are a few other components, such as an optocoupler and a few resistors to provide isolation and level-shifting. The data signals transmitted via this port are essentially RS-232 serial signals, at 31,250 baud with eight data bits, one stop bit, and no parity bits per frame. The USARTs built into the ATmega1284 can directly process this kind of signal, so the data line from the MIDI-In port is connected to one of these.

At present, the breadboard version of the ApOPL3xy includes a single port for MIDI-In, but I will likely add a MIDI-Thru port as well. MIDI-Thru ports are ports which simply output whatever signals came in on the MIDI-In port. They’re also pretty simple electrically, and require no software support in the microcontroller, so it’s probably worth adding one, just in case it turns out to be useful.

Conclusion

This covers almost all of the hardware used in the ApOPL3xy. I did leave out the amplifier circuits, because I’m still fiddling around with their design. Once I sort that out to my satisfaction, I’ll likely make another post about that. As always, I hope this has been interesting and informative. Let me know if anything is unclear, or if you’d like more detail on anything in particular.

I’ve added some new features to the ApOPL3xy synthesizer project I talked about in my last post. Some technical details follow, but first, let’s check out what the new version can do. Specifically, MIDI files can now be played from a computer (or other sequencer) into the ApOPL3xy to be synthesized.

Demo Songs

Here are some example songs. These songs were recorded by playing MIDI files from a computer into the ApOPL3xy’s MIDI-In port, running its audio output into the computer’s line-in jack, and capturing the line-in signal with Audacity. None of these files were post-processed in any way, other than to crop off leading and trailing silence. Links to the source MIDI file and the recorded MP3 are available for each song.

Harold Faltermeyer – Axel F (Beverly Hills Cop Theme) [MIDI] [MP3]

Toto – Africa [MIDI] [MP3]

Alan Menken – Under the Sea (from The Little Mermaid) [MIDI] [MP3]

Europe – The Final Countdown [MIDI] [MP3]

Finally, I would be remiss not to include this masterpiece [MIDI] [MP3]

New Features

Non-Volatile PATCH Storage

In my previous post, I discussed the Fat Man’s General MIDI 2-operator and 4-operator OPL3 patch sets. I’ve added the patches from the 2-operator set into the system, so they’re readily available without laboriously entering them using the UI.

They’re stored on an EEPROM chip (Microchip 25AA1024) for non-volatile storage, and cached in an SRAM chip (Microchip 23LCV1024). From there, they can be loaded into the ATmega1284’s system RAM as needed and sent to the OPL3 synth chip for use. I updated the patch editor UI to allow the user to select which patch to edit, and whether to save the edits in SRAM only or also in EEPROM.

OMNI MODE

Previously, the ApOPL3xy only processed MIDI messages on MIDI channel 1. Messages from other channels were ignored. This was enough to get the keyboard controller to be playable, but to play multiple instruments simultaneously, a synthesizer needs to be in omni mode. In this mode, the synthesizer processes messages from all 16 MIDI channels.

I’ve implemented support for omni mode in the latest version. (In fact, omni mode is always on now, as I have not yet added a UI option to turn it off.) A separate patch selection (a.k.a. “program”) can be made for each channel, allowing multiple different but concurrent instruments to be synthesized.

Soon, I plan to add a UI utility to select patches for each channel, but for now, I’ve implemented handling for the Program Change MIDI message to allow an input MIDI device to make the selection.

PERCUSSION CHANNEL

I’ve also implemented a percussive channel mode, which is commonly used for MIDI channel 10. This mode is special because the patch that gets used when a note is played depends not on the channel program setting, but on the note number itself. For instance, note 35 (B0) is for an acoustic bass drum sound, and note 40 (E1) is for an electric snare sound.

The Fat Man’s patch set includes a bank of percussion sounds for this purpose, and I’ve uploaded them to the EEPROM as well. I haven’t yet updated the patch editor to be able to edit these, but I will soon.

Note that this is distinct from OPL3’s percussion mode, which I also plan to support in the future. This mode was not often used, and in fact, none of the patches in the Fat Man’s sets use it. It’s therefore a lower priority. But it’s something the chip can do, so I want the ApOPL3xy to support it.

Update / Bug Fix

I was looking through the code yesterday, and I found a bug with pitch bend that would cause a pitch bend on one channel to bend new notes on all channels. The careful listener might have noticed something off with Africa and The Final Countdown. I’ve since fixed it and uploaded new recordings for the two affected songs, but I’m putting the original broken versions below for posterity.

Toto – Africa (Pitch Bend Bug) [MP3]

Europe – The Final Countdown (Pitch Bend Bug) [MP3]

It’s been a while since I’ve posted anything about electronics projects. I have not been idle, however. My current project is finally starting to come together enough to be able to show it off a little. It’s a MIDI synthesizer powered by the Yamaha YMF-262 (a.k.a. OPL3) FM synth chip, the successor to the YM3812 (a.k.a OPL2) I’ve talked about previously, and the Microchip (née Atmel) ATmega1284, a bigger brother of the ATmega328P used in the Arduino Uno. I’m calling the project “ApOPL3xy”, because I think apoplexy is a cool word, and it lets me put the OPL3 string in the middle.

Demo

I just got the patch editor working yesterday, so I’m excited to be able to show it off. It’s a bit rough around the edges, but it’s the first time the ApOPL3xy has felt like a real(ish) synth. Note that I don’t have 4-operator patches working yet, and there are a lot of menu items that you’ll see that don’t actually do anything. But 2-operator patches work.

I downloaded a 2-op patch set for the OPL3 authored by The Fat Man and used by the ADLMIDI project. I also used OPL3BankEditor to extract the parameters I needed to enter into the ApOPL3xy. The following videos show me entering in a few of these patches and demoing them with a MIDI controller keyboard.

Here’s an example of pitch bend working. This was a lot more difficult than I expected, and the details deserve their own post, but it seems to work pretty well now.

I also implemented a VGM player. VGM (Video Game Music) is a file format that stores commands to send to various synth chips. People have captured songs from lots of retro video games and uploaded them to vgmrips.net. I downloaded several, uncompressed them, and put them on the SD card for the ApOPL3xy to read. Here are a few examples:

Features and Limitations

There’s a lot of work left to be done, but the core is working. Current features include:

• MIDI Input
  • Currently limited to MIDI channel 1
  • Currently, the only messages supported are Note On, Note Off, and Pitch Bend
  • Velocity is not yet supported
• 2-channel audio output
  • The final product will have 4 output channels, but the breadboard version only has 2
• 18-voice polyphony when using 2-operator mode
• 10-segment LED VU meters per output channel
• Gain control potentiometer knob per output channel
• Menu-driven user interface
  • 20×4 LCD character display
  • 2 rotary encoders and 10 push buttons
• Micro SD card reader
• VGM player
  • Both OPL2 and OPL3 VGM files are supported
  • VGM file must be uncompressed. VGZ files are not supported.

There are several things I plan on adding in the future:

• 4-operator patch support
• Patch bank – currently the system only knows about one patch at a time
• Persistent patch/bank storage on EEPROM
• Import/export of patches and banks to SD card
• User interface improvements
• Support more MIDI messages
• Support MIDI velocity
• MIDI through output
• MIDI omni mode
• MIDI file playback
• VGM / MIDI file playlist support
• Printed circuit board
  • Split into two boards: the OPL3 board and the MIDI board. They’ll be connected with an IDC ribbon cable I think. This will allow the OPL3 board to be connected to other things in the future, like my homebrew 6502 computer, or a Commodore 64, etc.
  • ISP and JTAG headers for programming/debugging and headers for SPI, I2C and unused GPIO pins. This project was intended from the start to be hackable and upgradable.

Development Environment

I started out using the Arduino IDE to write the code for the firmware. That’s fine for small projects, but this quickly outgrew it. I recently switched to PlatformIO, and that’s been much more pleasant. It’s an extension for Visual Studio Code, and it supports a large number of microcontrollers and libraries. I’m using the AVR-ISP mkII programmer to upload the compiled firmware to the microcontroller. To access the micro SD card, I’m using the excellent SdFat library.

Closing Thoughts

This project has been a much larger undertaking than I initially envisioned. I’m excited that it’s starting to come together. I may post other articles about various technical details of the design and implementation another time, if there’s interest. When I get the PCB’s made, I plan to make the CAD files for the boards and the source code for the firmware available so people will be able to build one and mess with it if they want to.

The first step was to find some sheet music for the song. I found this, but MuseScore wouldn’t let me download it without registering for an account. I’m not about to do that. But, it’s pretty short, so I just entered it into my local install of MuseScore. Here it is for the curious:

And this is what it sounds like, in case you don’t remember:

Listening to this, I felt like it was pretty plain. I wanted to spice it up a bit. So, I added some drums (some tweaks were suggested by Donnett). Also, a retro video game shouldn’t sound like a piano. A square wave would be much more appropriate. With those changes, we get this:

That’s more like it! Much more fun to play Tetris to, IMO. But, it needs just a little bit more. Donnett has been teaching me some music theory, and I wanted to see if I could figure out what chords would sound good in a bass line. With one slight tweak from her (change first chord in second phrase to a 2 instead of a 7), I’m really happy with how it sounds in MuseScore (the chords start at 0:42):

For the musically inclined, here’s the updated sheet music and MuseScore files:

But we’re not done yet! Remember, the point of this whole exercise is to get it running on a 6502 computer with an OPL2 sound chip. To that end, I started looking at tracker programs. I found one called Adlib Tracker 2 that I liked pretty well, and set it up in DOSBox. It’s got a fantastic 90’s-era look, and it’s pretty powerful.

After a little learning curve, I was able to pick out some instruments, tweak them to get them sounding the way I want, and enter the notes. Here’s a video capture of the tracker playing the OPL2 version of my arrangement. (Note that the tempo is a bit slower. I didn’t notice when I was first doing this, but now listening to both speeds, I think I prefer the slightly slower version.)

The last step is to get this ported over to run on the homebrew machine. This was easier said than done. Eventually, I’ll probably write something to process the A2M file format created by Adlib Tracker 2, but this time, I used DOSBox’s OPL capture feature to capture a DRO file. Then I downloaded and compiled the utilities in the vgmtools github repository. It has two utilities I needed: dro2vgm, and vgmtrim. I had previously written a python script to extract the OPL2 commands I need from a VGM file. (By the way, vgmrips.net has a fantastic collection of music from old video games, using OPL2, OPL3, and many other chips.) So, finally, I have this song playing on my homebrew computer!

Pretty good for a 47-year old CPU!

I’ve made some changes from Ben’s design. I’m using CPLD‘s for address decoding, an 8k ROM instead of 32k (but I can adjust the address decoding to support up to 32k ROM), 64k RAM (not all addressable due to ROM and I/O), 4 65C22 VIA’s, and a 4 MHz clock. So far I’ve added a 20×4 character LCD, a YM3812 (OPL2) sound chip, and a NES controller. I have plans to add a PS/2 keyboard, an SD card reader, upgrade the clock frequency to 8 MHz, upgrade the OPL2 to an OPL3, and add a composite video display using the TMS9918A video chip.

I’ll make a bunch of posts in the future about various technical topics related to this, but I wanted to start with this general post introducing the project.