After some time off for the wedding, I’ve started back on the track to build the DBNZ CPU. It’s going to involve a level of complexity I’m not ready for yet, so I’m building the components piece by piece to get familiar with them.

First up is clocks. Here’s a really simple one based on an inverter with a Schmitt trigger, such as the 74HCT14.

If the input to the inverter starts low, then the output is high and slowly charges the capacitor through the RC network. The output stays stable during the charging due to the Schmitt trigger, but once the upper threshold voltage is reached it flips, and the capacitor begins discharging. Eventually it hits the lower threshold, and starts over again.

The frequency depends on the lower and upper thresholds, which according to the datasheet are about 1.8 V and 2.8 V when VCC is 5 V.

Here’s a clock that uses a 555 timer and has a reset input.

Bring the reset# input low to immediately bring the clock output low and keep it there.