The Master Clock

The master clock needs to run at 12MHz. The easiest solution would be to use an oscillator module. Plug in 5v and out pops 12MHz, job done ! However, not having one to hand, I use an alternative using the 74HCU04 device. Note the "HCU" in the part number ; this specifies an unbuffered device specifically targetted at producing oscillators, so don't try using the more common "HCT" etc types of devices.

Although the component values worked with an initial 6MHz crystal, the 4K7 resistor had to be reduced to 2K2 in order to establish oscillation with a 12MHz crystal.

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