Clock and Reset

A project log for wirez80

Z80 machine with a hex keypad and 7-segment displays built on a cylindrical wireframe

matsengmatseng 01/07/2019 at 17:140 Comments

I'm getting a bit stressed now, it's only a bit over 24 hours until the deadline and I haven't even tried the CPU yet.

One of the most important things needed to get it running is the Clock and a Reset unit.

So a cobbled together a small two-chip device with a 74HTC04 (hex inverter) and a 74HCT393 (dual 4-bit counters) for this.

Half of the '04 is a used for the Reset-button and driving the reset LED. The other half is a 7.37Mhz crystal oscillator which is the same frequency the RC2014 uses. 

Since the wiring of the bus is a bit "unconventional" and also have a 1 Meg resistor between the individual lines I'm not fully convinced that I can run this at the 7 MHz clock frequency. So I added a 4 bit binary counter after the oscillator so I can select either 7.37 Mhz,  3.68 MHz,  1.84 MHz,  921 Khz or even a slow 460 KHz.

If I really wanted to go even slower I could hook up the second part of the counter) to gen all the way down to 28 KHz but I see no real reason for that. Then it would be more fun to have a 10 Hz clock rate to really get the LEDs blinking in a rate that's visible.

Clock and Reset unit
Clock and Reset unit
Clock and Reset unit backside
Clock and Reset unit backside