The Z80 Embryo is the start of my homebrew computer. Initially, I was going to go in deep and complex…. but perhaps its better to do small steps and learn as I go along.
The Embryo is simply the Z80 Tester on a breadboard to look at the starting sequences of the chip and to test that the chip essentially appears to work.
The first step was to build up the clock testing circuit. This design is Ben Eaters design although I changed the switching circuit as I didn’t have a toggle switch that would work well for this. I, instead, used a push button together with the 555 to create a toggle. This is a great design for CPU testing – mainly for testing and operating a homebrew processor but just as useful on a homebrew computer.
The circuit is split into 3 main section.
1. The adjustable frequency clock
2. The One Shot pulse
3. The control circuit to switch between the one shot and the clock.
Each section is using a 555 timer. You could user a variety of other latches and components, but like most people, I had a few 555 lying around so there was no need to go out and buy any new components.
Section 1 – Adjustable Frequency
This section is a simple timer in an astable mode, meaning that it toggles the output state. A Variable resistor (potentiometer) is used to alter the charge/discharge time of the pulse from about 1Hz to *** 300Hz.
Section 1 – The one shot
This section includes a push button switch to trigger a controlled single pulse. This is great when you need to step through the clock sequence one at a time in order to see and or measure what is going on with the processor.
Section 3 – The control and switch
As mentioned earlier, I changed the design a bit here, In Ben’s version he uses a toggle switch and the 555 timer is configure to de-bounce the switch. I didn’t have a toggle switch that would fit nicely into the breadboard, and I thought it would be good to use a push button to toggle the state of the output when the button is pushed. So this 555 timer is configured to trigger or reset based on the current output of the 555, thus enabling a push button toggle switch.
The output of this 555 is then also fed into control logic to allow either the One Shot or clock though to the final output.