Reverse-engineering the 1977 Unisonic 21 calculator/game (part 1)

My Unisonic 21 (S/N 026255) in game mode.

My Unisonic 21 (S/N 026255) in game mode.

My Dad bought this [Unisonic] 21 calculator/game in 1977, when I was eight years old. It is a four-function calculator that could also play [Blackjack ("Twenty-One")]. I played with it incessantly, and was able to intuit most of the rules of Blackjack from it.

Perhaps more infamously, the Unisonic 21 was marketed as "signed by [Jimmy the Greek]", a sports commentator and bookie who was not a nice guy.

Anyway, the inside contains a single 42-pin [QIL] chip labeled A4821. This is a mask-ROM [PPS-4] Rockwell CPU, a 4-bit PMOS processor. [Datasheet]

The insides. In the center is the A4821, date code 48th week of 1977. The VFD is toward the right, and the round-looking thing next to it is actually a DC-to-DC converter to provide -14.5v to the chip, and -31v to the VFD. The PCB on the left is the keyboard.

The insides. In the center is the A4821, date code 48th week of 1977. The VFD is toward the right, and the round-looking thing next to it is actually a DC-to-DC converter to provide -14.5v to the chip, and -31v to the VFD. The PCB on the left is the keyboard.

The system was patented in 1977 in [US4339134]. Interestingly, the patent contains the entire assembly language listing and flowchart for the program.

Since I'd been fairly successful in identifying various CMOS circuits in the [HCF4056 reverse engineering], I thought I'd attempt a more primitive but more complex chip, and what better target than the one in my favorite handheld game from the 70's.

I sent three of the A4821 chips (these games are cheap on eBay) to [Zeptobars] for decapping. I also visited John McMaster at [siliconpr0n] to learn about decapping first-hand, and we also decapped the A4821. I'm really interested in comparing the two results!

I'm still working on this, it will take quite a long time. But I've identified various bits, and this will serve as a sort of visual orientation guide to the chip.

I find it interesting that on the web there are plenty of side views of VLSI components, but virtually none of top views from actual dies.

This one is the easiest to identify, the metal layer.

Metal wire.

Metal wire.

This next example is a diffusion wire. I think the outline is caused by the material used to mask off the part of the chip that did not receive diffusion. In any case, it's very convenient.

P-type diffusion.

P-type diffusion.

The next example shows a contact or via between metal and diffusion. The diffusion ends in a square, as does the metal. There is a bold inner outline, and then you can barely see a very thin outline inside of that. The thin outline indicates that the metal goes into the diffusion, forming a contact. I have no idea at this point what the bold outer outline is. Maybe it's just metal, meaning that the contacts were formed in a step prior to the metal wire layer. The side view shows my hypothesis.

Contact.

Contact.

Transistors kind of look like bandages. The thin outline of the rectangle going vertically is a layer of oxide. It touches metal on the top in the side view, and straddles diffusion on the bottom in the side view.

PMOS transistor.

PMOS transistor.

A capacitor is formed between diffusion and metal with the oxide insulator between. You can see the oxide come in from the upper right, and basically surround the diffusion area of the capacitor. What makes this different from a transistor is that the oxide doesn't straddle a continuing section of diffusion. The diffusion sort of dead-ends.

Capacitor.

Capacitor.

Most of these things have their metal layer connected to VDD (-14.5v). They have no oxide, so they are not capacitors. I think they are resistors. The inner bold outline must be some kind of resistive material.

Resistor?

Resistor?

In the die marking, it's interesting that there are two underlines after A48. This must mean that the masks were standard, and only the ROM section of the mask was changed. The A48 part and the dot after the number appears to be diffusion with metal on top. The underlines are metal only, while the 21 part is diffusion only.

I'm not sure what AFFEIO means or stands for, but it seems that the A is diffusion, the first F is oxide, the I is metal. The other three are unknown. Perhaps one is the resistive material.

Die marking and tests.

Die marking and tests.

Some other test patterns: