Vtech Whiz Kid reverse engineering

In an effort to free up space and get some low hanging fruit projects done fast I took on this one. This is a small learning computer from the 1980s. It uses a custom segment LCD for display, an optical barcode card reader and cartridges for data, a regular speaker for sound, and a membrane keyboard for input. I decided to go fairly in depth reverse engineering the hardware even though the software (and even the architecture of the processor) is still a bit of a mystery.

The cart is a pretty good place to start. I generated an entire schematic of this but the highlights are that it’s a 4-bit interface and the jumpers can pick from different pinouts of ROM chips. J7, J8, J9, and J10 pick which 4 bits are sent to the card edge, since the most common ROM chips are 8 bits and this machine is apparently 4 bits then they only need half of the ROM. The other half of the mask ROM is probably a different program and would be sold with the jumpers in the other configuration for that expansion pack. J11 and J12 swap which pin is A11 and which is an unused chip select pin, this allows a TMS4732 like is present right now or a more standard 2732 to be used, see my ROM pinouts for the slight differences. The other interesting thing is the use of a PNP NPN pair of transistors to cut power to the chip unless the chip select is asserted. This is clearly to cut down on power consumption of a battery powered toy, but thinking back on it now, why don’t ROM chips do this internally to save power? There’s also an unpopulated one for if a 2732 is fitted.

The next part I reverse engineered was the barcode reader. It uses an LM358 to amplify and digitize the signal from the optical sensor so the data can be read from the cards. There is only one track so you have to pull the card at just the right rate, but I was never able to do it consistently. It’s possible the part values have drifted far enough that the pot onboard couldn’t bring the signal back in spec, or maybe I’m just bad at it.

The screen is quite primitive, it’s an LCD segment display with some segments to create animations (like the game and watch series) but also some starbursts for character display. I managed to get a picture of the whole thing lit up due to some sort of bug when I had the keyboard disconnected. This should help if anyone in MAME can get a virtual set of segments drawn up. Higher quality versions of the back transparency and the rest of these images (as well as the cards and rom dump) are hosted at the internet archive.

Next I generated a schematic for the entire mainboard. This took a while but was not that difficult, it is only a one sided pcb with a lot of jumpers. The main takeaway is that the processor (which has to have internal ROM for the default game) also has the ability to drive the LCD directly. It has a parallel bus for reading the keyboard and the ROM from the cart, and the power button on the key matrix is a soft power switch at the hardware level.

My pin numbering scheme is not that inscrutable, is it?

The last thing of importance is the main processor itself. It is labeled T7826 with a date code of the 36th week of 1985. It can drive the LCD directly, it seems to have a 4-bit data bus and at least 11 bits of address space. The oscillator is either internal or an RC oscillator on pin 73. It uses a QFP92 package. From my research this is likely a Toshiba tlcs-42, 46a, or 47 series 4-bit microprocessor but I’ve never seen a datasheet for one in a package grater than 80 pins. I went one step further though and sent it in for decapping, having learned just about all I needed to know from the working machine. There hasn’t been a conclusion yet regarding the ROM bits or anything else, but maybe someone out there can verify my pinout findings or the processor architecture from the die.

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