First Power Up

A project log for TD4 CPU

TD4 CPU is a "4bit CPU that runs". It is very interesting for what can be done with so little.

agp.cooperagp.cooper 09/20/2017 at 07:150 Comments

First Power Up

Sorry, it did not work!

Address 0-3 light up concurrently

Tracked this down to duplicate labels RA0-3 in the schematic.

Fixed by cutting tracks (8) and adding wire links (4).

Jump does not work

The problem was that the diode ROM low voltage is about 0.9v but the 74LS14 needs less than 0.8v to trip. A 74LS04 would work (trip voltage is about 1.3v) but better to use a 74HC14. I am migrating to HC anyway.

Read/Write to RAM does not work

What can I say, I did not check the schematic and it does not work!

Removed the RAM and support chips.

Okay, the basic TD4B now works (except for the memory):


Use different coloured LEDs, it adds to the visual appeal of the CPU.

I also have to redesign the decoder, !WE is flawed and the next step is to add a Page register.

Also the AOUT and DOUT registers should be reset on RESET.

At the moment the the TD4B is programmed to count from 0 to 255 and then stop: