The AY-3-8606, another successfully simulated circuit

Since I began this blog it has focused on preservation of the AY-3-8500 "Pong on a chip" circuit (with one exception.) As has been said before, the tools and experience developed to emulate the AY-3-8500 can be used on other discrete circuits from the time period. The AY-3-8606 "Wipeout" game has now been simulated using said tools and experience, just like the '8500 was in August, and it took under two months to do so. This post describes the work done so far, and also documents some of its internals.

How to emulate discrete integrated circuits

I've talked about the process involved in past posts, here are the steps laid out again.

1. Obtain circuit specimens
2. Decap, photograph, and stitch images
3. Highlight images
4. Process images and correct errors
5. Simulate circuit in JavaScript, correct more errors
6. Run DLAET to abstract netlist into Verilog
7. Port to FPGA
8. Use Verilator to emulate circuit in MAME

At this moment, the AY-3-8606 is at step 5 and the AY-3-8500 is currently pioneering step 7. Thanks to Sean Riddle for undertaking steps one and two, which resulted in the images below. Note: All die photos on this post come from Sean Riddle under CC-BY-4.0.

You can check out the JavaScript simulation here. The simulation is purely for debugging purposes, so I haven't made much of an effort to make it user friendly. The files involved with the process are available here.

First impressions

The AY-3-8606 was part of General Instruments 86XX "economy" game system circuits. Like the earlier AY-3-8500, GI sold these chips to anyone wanting to build and sell systems using them. Many almost identical consoles were produced using the schematics and screenshots found in the catalog (page 460), these are now referred to as PC50x systems. The console contains the support circuitry, controls, color chip, modulator, speaker, etc while the interchangeable cartridges contain the unique game chips.

A standard PC50x system (pong-story)

The PC506 game carts went by "Wipeout", "Wipe-off", "Destruction Game", "Brix Game", or "Jeux de destruction" and contained an AY-3-8606 with it's pins mapped to the cartridge connector. 10 game variants, some two-player, can be selected using the console's buttons. Additionally, three switches could adjust the bat size, ball size, and speed of the game. Below is a table comparing it to the earlier AY-3-8500. It's a bit more complex, but not by much.

	AY-3-8500 (NTSC)	AY-3-8606 (NTSC)
Transistors	2318	2975
Nodes	973	1633
Polygons	10292	12435
Input clock	~2Mhz	3.579545Mhz

The AY-3-8606's surface.

Above is an image of the decapped chip's surface. There are a few noteworthy features immediately visible. Three wafer test circuits are present on the edges of the die. The text "© 1978 G.I. CORP." fills an empty spot in the upper-left. Nearby that is a collection of numbers written using different layers. Lastly "80-80395" appears in the bottom-right. None of these features affect the game logic, so highlighting them isn't necessary.

There is a repeating structure near the bottom right. This stores the state of the blocks inside six shift registers, corresponding to six rows of blocks. Interestingly, there are 10 segments per shift register, but only 8 blocks per row. More on this later.

Fixing errors

 

A high resolution look

The 2048*1810 images I used are clear enough to discern almost all features on the chip. The indistinct spots are resolved using the high-resolution images. Nevertheless little errors are made and need to be found. Four errors had to be fixed before my image processor would succeed. After it did the debug images then revealed over two dozen other errors. After a little more debugging work and modification to the virtual TV code, the JavaScript simulation produced the image below.

The generated image, along with the datasheet's rendering of game #9

It works! ...but not correctly. It looked like game #9 from the catalog, except some of the bricks were on the wrong side, the scores were vertical lines, and the bricks overlapped the top bounds. Time for more debugging!

Unlike some other chips, PC50x series games use strobed signals to read up to 10 different game select buttons with only 7 pins. It seemed that without external buttons the circuitry was changing to game #10 partway through a screen, explaining the misplaced bricks. I wrote a little code to simulate the external buttons via a checkbox, which fixed this issue. Because of this strobing the 8606 will probably exhibit odd behavior whenever multiple buttons are pressed, I haven't experimented with it though.

After selecting one of the other games, I saw that 9 columns of blocks appeared instead of 8! A close look at the shift registers revealed that they held 10 columns of blocks. The 10th column, which is off the right side of the playfield, is always reset to empty. The 5th column (middle) is reset to empty/full based on the game selection logic.

Take a look at this captured footage (skip to 4:14.) In game #9 the wall appears to be 9 layers thick (this confirms the catalog's screenshot.) So at least one game mode uses the middle column. It turns out that activating the reset pin (which the simulation does before loading) can only reset the blocks when the scanline is in a certain region. If you turn it on for longer it will correctly reset the blocks. This means that switching from game #9 to another one without hitting reset will leave blocks in the middle column, doing the opposite will create a gap in the middle of the wall. Also, the footage seems to suggest that the blocks do overlap the top boundary in real hardware. I don't have a physical console to confirm that this is the case, but the fact that little details like these are preserved is pretty impressive in terms of emulation accuracy.

This was causing the vertical lines. Can you see what's wrong?

The right player's paddle and score were missing (although I'd personally be fine competing against someone with no paddle), and the left player's score was still vertical lines. After a little more error hunting the output looked correct for all possible games. Odds are there is still an error or two present, but most of the circuit is correct.

Game 7

Looking at the circuits

I've explained some of the AY-3-8500's circuitry in great detail. My current focus is emulation, so I won't do the same thing for the AY-3-8606. Here's a short overview of different components based on what I learned while debugging it.

The 3.579545Mhz clock signal comes in through the aptly named clock pin at the top. This is halved and used to drive two alternating clock signals throughout the chip. 3.579545Mhz is the NTSC colorburst frequency, crystal oscillators at this frequency were mass produced for TV sets and thus made inexpensive by economies of scale. The non-NTSC 86XX variants, many later systems and even non-television chips used this standard frequency as a clock source or reference.


Next to the clock divider + driver is the horizontal and vertical control signals, just like in the AY-3-8500. The shift register is a little smaller though. All of the shift registers in the 8606 are more compact because they use capacitor-capacitor segments, unlike the AY-3-8500 which uses mostly capacitor-latch segments.

To the left of that is another horizontal+vertical counter pair. What this one does is a mystery to me, although I''m guessing it's part of the ball circuitry. Past those are the two paddle counters, one for each player.

In the lower-left is the score counter/display logic. It seems almost identical to the score areas on the 8600 and 8605 die photos. Another area of interest is the center where a game select PLA resides.

Next Up

The next step for the AY-3-8606 is to convert it to Verilog using DLAET. That will have to wait until I get the AY-3-8500 fully working on an FPGA. In the meantime, I'll be working on highlighting another chip.

You can contribute by highlighting! It's visual identification, not necessarily hard, it just takes a lot of time. The AY-3-8606 took over a month to highlight, while processing and error hunting took less than a week. Highlighting doesn't require much focus, so I worked on it whenever I needed to listen to something else. Shout-out to all the people who let me work on a laptop wherever one does not usually work on a laptop, you are preserving games you never even heard of!

What chips are next? plgDavid of the MAME team is sending off some chips to be decapped. He has the PAL variants of the 8603 (road race), 8607 (light gun games), and 8610 (improved 8600.) Hopefully useful die shots will show up sometime soon. If you have circuit specimens or the consoles that contain them (nonfunctional included) and wish to donate, I'll help with finding someone to decap them. I'm particularly eager to get die shots of Atari's chips, and the ones in Nintendo's Color-TV game series.

In the meantime, there are four existing die photos I'm looking at for emulation. First up is the 8600 (NTSC variant) which is pretty much an upgraded AY-3-8500 with more games and bidirectional movement. Second is the 8605 (PAL variant) submarine/warfare chip. Third is the MM57105, National Semiconductor's competitor to the AY-3-8500. It could play its games in COLOR (without a support chip.)

Die surface of the TMC0280

The last game chip to have been decapped as of 3/2019 is the MPS-7600, from the Coleco Telstar Arcade cart #1. I'm not doing this one anytime soon due to lack of any documentation, and also because its much more complex than what I've done so far (it has a custom microcontroller inside.) The fourth chip I'm thinking of emulating is the TMC0280. Never heard of it? How about the Speak & Spell? The TMC0280 is the voice decompression/synthesis engine inside it. It has a lot of transistors, but many of those are part of repeating arrays.

So which one to work on next? Comment below if you have a preference, or try highlighting one yourself. I'm leaning towards the AY-3-8605 because its a fairly unique game.

Two other things: first, I'm in the process of setting up a Patreon account for anyone wanting to contribute monetarily (all files and posts will still be public.) EDIT: here it is! Second, I'm going to be giving a talk at Latchup conference in Portland (Oregon) come May. Stop by if you're one of the 0.0086% of people around there. Anyway, my next post should be on the current/future progress of FPGA porting, so stay tuned (to channel 3 or 4. ;)

I ran the simulation for 10 million cycles while writing this (~1.4 seconds) The ball clearly moves, bounces and destroys blocks. (There was an error in the score logic)