CPU clock module (with single step)

Simplified version of Ben Eater's Clock Module

For experimenting with retro CPUs like the 6502 and the Z80, this is a very handy clock module. It can single step and it can run free. Like all Ben's projects, his Clock Module is intended to live on a breadboard.

I use this clock like a tool though and I'd like it to be stable, reliable and portable which means I wanted to get it off the breadboard and into a more permanent state.

This process got me thinking:

• can I reduce the chip count and the size of the device?
• can I make it run slightly faster than its default peak of ~500Hz?
• what don't I need?

This resulted in a design where I reduced the chip count from 6 to 3 and increased the peak speed to ~1kHz. Part of this optimization was to ditch features that are probably not used outside of Ben's TTL CPU like the halt option and the inverted CLK output.

Modifications

Replaced two of the three NE555's from Ben's module with a single NE556.

Replaced R2 (1K) in Ben's schematic with a 220 ohm resistor changed the upper bound speed from ~500Hz to ~1kHz.

Ditched the halt feature and the inverted clock output (highlighted in pink below). This reduced the output selection logic from three 74xx series chips to just one, a 74LS00 quad 2 input NAND gate chip.

Perf Board Version

Having made and tested these simplifications and modifications, I moved my version of the clock module to some perf board. It has long leads on D1, SW1, SW2 and RV1 which will be clipped once I've mounted them in the face panel of my case.

There are 3 interface pins for 5V (red), GND (black) and CLK (yellow) respectively.

3D Printed Case

Next we need a sturdy little case for our clock.

I usually make a template to test the fit for holes when mounting knobs and dials. Much quicker and more resource friendly to prototype with a throw away template like this than printing the entire case at full resolution only to get it wrong.

Choosing which surface goes on the print bed and then being careful about gradually increasing the slopes of the overhanging parts results in a design that needs no support material when printing. The output at high resolution then needs no clean up or finishing work.

Result

Now I have a clock module that is portable, compact and rugged.