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CPU clock module (with single step)

Simplified version of Ben Eater’s breadboard clock with chip count reduced from 6 to 3. I use this clock with my 6502 and Z80 projects.

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.

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.”

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