GoKawiilI bought a cute little 4 bit cpu kit from Aliexpress called the TD4. It has 2 registers, some LEDs, and 16 bytes of program ROM. Quite limited but still very cool and teaches a lot of principles of computer architecture.
The documentation, schematics, and pictures for this cpu are here https://github.com/wuxx/TD4-4BIT-CPU. It’s a little sparse though. I can imagine a student getting overwhelmed. So I thought it would be helpful to write some longer form notes.
Building
We took two sessions just soldering it up.
This image
plus the schematic were enough instructions such that it wasn’t too hard.
The directionality of the surface mount diodes gave us pause, but we used a diode tester functionality in our multimeter to figure it out. The tiny green line on the front of the diode is towards the bottom of the board. I believe the line was on the back of the diode was towards the top, but it is soldered down now so I can’t check.
The thing that gave us the most trouble was soldering on the USB connector. I’d advise doing this before putting on the IC sockets, because it got in the way of the iron. The middle pins of the USB are unconnected, so you can just blast them if need be. The USB is only for power.
Which chip goes where can be seen by inspecting the schematic for part number and IC number. The notch aligns with the notch printed on the PCB board. It basically worked the first time, minus an intermittent power connection on the usb.
Soldering all those diodes sucked.
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