SAP 1 Clock module.

I've recently been watching Ben Eater's series on his breadboard 8 bit computer. I have now started my own, but as suggested by Ben I will be making some changes.

Firstly I've built my clock module.


1 is a NE556 - a dual 555 timer, which provides the astable and monostable timing signals. The blue push button is used to provide single step pulses. 2 is a single 555 used to debounce the monostable/astable switch.

Chip 3 is a 74LS14 hex inverter with Schmidt trigger inputs (similar to a LS04, and I have quite a few)

I have replaced Ben's 74LS00 and 74LS32 with a single 74LS51, which contains 1 2-wide 2 input and-or-invert gate and 1 2-wide 3 input and-or-invert gate.

I've used the 3 input gate to provide the halt signal, by feeding the inverted halt signal to the extra input I remove the need for the final and gate in Ben's design.

I was also left with an unused and-or-invert gate, which I have used to provide a single step function. By feeding the output from the clock run/single step switch through one of the and gates on unused part of the LS51. The other input is fed from an inverted single-step input.

So I can tell what mode my computer is running in I've added a bi-colour LED just below the switch. This glows green when the clock is running, and red when the clock is in single step mode.
I will find uses for this once my computer is built.