Interfacing with a Ben Eater 6502 computer

A project log for World's Simplest TTL VGA circuit?

How many TTL ICs do you actually need to output a VGA image?

George FootGeorge Foot 10/19/2020 at 20:300 Comments

Since publishing the original ROM-based version, I've looked into hooking it up to a Ben Eater-compatible 6502 computer.

This went really well - so I've added the schematics to the project gallery alongside the more compact ROM-based one.  One shows the general setup, the other gives you three choices of clock timing for the CPU.

There are some demo videos linked below, but first I'll go over the key changes to the circuits...

Extra components

The only additional components are three transceivers, a quad 2-input NAND, and a dual D flipflop; and of course the ROM is replaced with RAM.

BE6502 changes

The changes you need to make to the BE6502 are:

  1. Disconnect the ROM's ~OE line from ground
  2. Disconnect the CPU's PHI2 input from the existing clock

Those two things need to be driven by ~ROMOE and PHI2 in the new circuit instead.

Video circuit changes

In the video circuit, compared to the ROM version - the ROM is replaced by RAM; the ~OE pins on the counters and RAM are disconnected from GND and joined together and driven by the new timing circuit; and the ~WE pin of the RAM is also driven by the timing circuit instead of being connected to VCC.

Demo videos

Here's the first video covering the hook-up:

And here's the follow-up where I made the CPU clock be driven by the video circuit, to get rid of all the glitches: