Thursday, September 20, 2018

A visual simulation of the SP0256 (beta)

Well... I got a little distracted over the weekend. For those that haven't been following me, I built a transistor level simulation of the AY-3-8500 Pong-clone System-on-Chip over the course of several months. For this September's RetroChallenge, I've been reverse-engineering chunks of it and posting my findings here. I'm sorry to say that I haven't worked on it at all since my last post, because instead of that I built a new simulation of a entirely different (and much more complex!) microchip; the SP0256.
The chip is HUGE! This is less than an eighth of it's surface

Well, its been a productive few days, here's what happened.

Along with the AY-3-8500, there are a slew of other antique chips I was interested in (e.g the AY-3-8600 series, the MPS7600, etc) and reading about, the SP0256 included. The SP0256 is an fascinating piece of circuitry, in short; its a speech decompression/synthesis chip. It was used in countless classic computer expansion cards, expansions modules for video game consoles, and an untold number of hobbyist projects. This particular one came from the Intellivoice expansion for the Mattel Intellivision. You can listen some sounds from it in the video below.

I would've love to mess around with transistor-level emulation of this chip, however the process of marking the components can take several months of work, and I think that I've done enough highlighting (for the AY-3-8500) already in 2018.

Then last Thursday I stumbled across this thread on AtariAge, where, lo-and behold another retro-geek had highlighted and cleaned up the entire SP0256 chip! Several months of work, already done, just ready to be turned into a simulation, thanks very much John PCAE! After finishing my last post on Friday I started creating a python script to remove curves in the image, which was partly unnecessary as it turned out. By Saturday afternoon I put the image into the image processor I built to process the AY-3-8500 (which I still need to clean up and release...), and after several errors were fixed, the program spat out segment and transistor definition files, which were actually larger than the source image.

Very late Saturday I put those files into a copy of my previous simulation code and turned the clock on and off. It didn't do very much at first. Toggle buttons and output displays tailored for the chip were added into the simulation. The next four days were filled with finding and fixing the bugs/errors in the source image, my python script, my image processor, and the simulation itself as well as learning more about the chip and the way it generates the sound.

And here it is, a (partially working) transistor-level emulation of the SP0256. Note the word "beta" at the top, there's definitely more work to be done. There are still plenty of bugs to iron out, chiefly the chip will not disable the instruction decoder while shifting a new byte in, resulting in several spurious instructions being (partly?) executed for every real one. Chances are that some wire didn't get overlain or processed correctly, I just need to find it (I've already managed to fix many other errors.)
You probably will have no idea whats going on (don't worry that's normal) I'll explain the workings more in-depth eventually, for now there are a few points of interest you can look at.

1. That big rectangle on the right is a 2 Kilobyte ROM containing a program to generate the generic phrases used by the Intellivoice, like "Mattel Electronics Presents". The chip supports internal and external ROM chips, such as game-specific ones inside the cartridges. The active selection column will change as the program progresses.
2. Here is a counter + decoder which generates multiple low level timing signals. If you let the chip go the control lines coming out to the right will pulse at regular intervals. Interestingly the lowest and third lowest signals are not connected to anything. The signal generator works almost exactly the same as the ones in the AY-3-8500.
3. The instruction decoder is here. Thanks to intvnut for figuring out which lines mean what opcodes, they are now named accordingly. The instruction set is listed on this page.
4. The clock circuitry is here, if not disabled by a low standby reset pin, it will advance the internal circuitry, as well as external circuitry through the ROM clock pin (lower left)
5. The chip outputs an analog sound waveform through a digital PWM pin (Pulse Width Modulation.) On the right hand side is a virtual oscilloscope trace of it, which currently only generates a repeating pattern due to stated bugs. Once the output starts working I'll add in a simulated analog waveform tracker, and possibly a way to export this sound to a file.

You may notice the simulation runs a little slower than the AY-3-8500 simulation, its a bigger chip (at least in complexity, not necessarily physical size.) A little comparison table is below
Logic Transistor Count
Physical Size
~31.36 mm2
Die Photo Size (pixels)
Clock Rate
Introduction Year

Early 1980s

Wow, I'm amazed I got this all done in less than a week! I'll admit it's not as polished due to the rush, that will be fixed eventually. Next I need to get all the errors ironed out so the virtual chip can actually produce a waveform. I'm still going to work on the AY-3-8500 sim and talk about it, specifically the score circuitry next. Lets see what I can do in the next 1.5 weeks!
In the meantime, comment or ask questions below (come on, at least something went over your head.) I'm announcing my posts on twitter so give me an upvote! (Oops, wrong site.) If you want to see someone sail a sailboat while using an Apple ][ to obtain GPS coordinates or something along those lines, head to the RetroChallenge entrants list. And now to end this post with a few words of wisdom...

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