CPU

The CPU

The heart of the Whizz80 computer is the CPU.

Z80 CPU

Control Signals

The control signals are tied high each with a 10k resistor. The Whizz80 computer doesn't use the /WAIT or /BUSREQ signals so they are permenantly set high.

Control Signals

A simple RESET circuit can be created with a push button and capacitor (shown below). You could also duplicate this circuit for either the /NMI or /INT to generate an interrupt. I have found that manually generating a /NMI signal is a nice way to continue/resume from a HALT instruction.

A simple /RESET circuit

Status Signals

To monitor the outbound control signals, you can simply place a LED and resistor to ground. It is very handy when stepping through some of your code to see the CPU status, but perhaps not so much when running the CPU at full speed. For this reason, you can disable the display of the signals by adding a jumper between the LED and the CPU. In my example below I have made the three status signals (/MREQ, /IORQ, /CLOCK) optional.

Status Signals

I've found that the most handy status signal has been the /HALT signal because it only lights up when the CPU is halted by the HALT instruction. It makes a handy hardware breakpoint during testing/debugging.