This project contains a simulator and primitive assembler for the TD4 4-bit CPU, see https://github.com/wuxx/TD4-4BIT-CPU for more information, or just look at the code in td4.c.

Check out the doc/ subdirectory for the original project.

Ideas / To Do:

I will most likely never get around to doing this, and instead just using this space to document some ideas.

• Translate https://github.com/wuxx/TD4-4BIT-CPU into English.
• Get this design up and running in Proteus Design Suite / Other PCB and circuit simulator software.
• Tweak circuit, make improvements, such as:
  • Better power supply input / alternatives.
  • Power supply over-voltage, over-current and reverse bias protection https://www.circuitbasics.com/protection-circuits.
  • Instead of output going to some LEDs, it could also be brought out to an extension board.
  • Instead of DIP switches the input could be (optionally) be taken from an external set of input pins.
  • Could the diodes on the board be replaced by LEDs to give more feedback? We could certainly light up the register output and which instruction is being executed.
  • Adjustable frequency control option.
  • Changes / improvements to instruction set.
  • Make the CPU more configurable using jumper cables (e.g. we could perhaps make the Jump On Carry into a Jump on Not Carry via a jumper, along with some of the instruction decoding logic which could be changed via jumpers as well).
  • The Holtek HT12A/HT12E Encoders and HT12D/HT12F decoders could be used in conjunction with some simple radio circuits to make radio controlled peripherals.
• GUI simulator
• Simulator in javascript using WebGL to display an interactive version of the TD-4.
• Turn this into a primitive Programmable Logic Controller.
• Make more example programs
• Improve assembler (add labels, debugger).
• There are "Configurable Multiple Function Gate" ICs, such as the 74LVC1G97 that can be used as a MUX, AND, OR, NAND, NOR, an Inverter or a buffer using 3-inputs (1-output). That device only allows 1-bit to be processed at a time (4 of them would be required) but it goes to show there are options for changing the instruction set. None of these devices are through-hole however, which is part of the appeal of this system.

Links

• https://github.com/wuxx/TD4-4BIT-CPU
• https://hackaday.io/project/26215-td4-cpu
• https://teardownit.medium.com/diy-4-bit-cpu-917e7bff228a

Some stuff on the Richard's Controller, kind of related:

• https://sites.ecse.rpi.edu//courses/static/ECSE-4770/LAB_3/lab3_rev_f.pdf
• https://www.docjava.com/cpe210/pdfs/richards.pdf
• https://en.wikipedia.org/wiki/Richards_controller