Clock

This module is controlling the speed of the entire computer. It generates a 50% duty-cycle clock signal that is used to synchronize all modules with each other.
A switch lets you select between the internal 1MHz clock and an external clock, which can be connected to the breadboard using a jumper cable. There’s also an input for a halt signal which, when pulled high, stops the clock until the computer’s reset button is pressed.

Schematic