I'm building an 8-bit CPU out of 74HC logic in KiCad — seven module boards, no
backplane, everything daisy-chained on three ribbon cables. This is the clock
module, the first board of the set and the one that has to be trustworthy
before anything else can be debugged.
Available for sale
https://www.tindie.com/products/spartatux/clock-module-dip/ **Two logic packages, 22 parts total.**
- **74HC4060** — crystal oscillator and 14-stage ripple divider in one package.
16 MHz HC-49 across pins 11/10, 1 MΩ feedback resistor across the same pins,
27 pF at the inverter input, 100 pF at the far end behind a 2.2 kΩ series
drive resistor.
- **74HC14** — hex Schmitt inverter. One gate buffers the raw 16 MHz off the
oscillator node, two form the step debounce and the output driver, one drove
the inverted output, and the two spares have their inputs tied to ground.
Everything else is passive: five caps, six resistors, an LED, three switches,
two headers.
## Eleven taps, selected with a jumper wire
An 11-pin header carries every available frequency and a single pin beside it
is the selected one. You pick the clock rate by moving a jumper.
| Tap | Divisor | Frequency |
|---|---|---|
| `OSC16M` (via HC14) | ÷1 | 16.000 MHz |
| `Q4` | ÷16 | 1.000 MHz |
| `Q5` | ÷32 | 500.0 kHz |
| `Q6` | ÷64 | 250.0 kHz |
| `Q7` | ÷128 | 125.0 kHz |
| `Q8` | ÷256 | 62.50 kHz |
| `Q9` | ÷512 | 31.25 kHz |
| `Q10` | ÷1024 | 15.63 kHz |
| *(Q11 not brought out by the '4060)* | | |
| `Q12` | ÷4096 | 3.906 kHz |
| `Q13` | ÷8192 | 1.953 kHz |
| `Q14` | ÷16384 | 976.5 Hz |
The frequencies are silkscreened beside the header so the board documents
itself on the bench. The 16 MHz tap comes off the oscillator node through a
Schmitt inverter, not through a divider stage.
## Or stop it and press a button
A slide switch picks between the jumpered tap and a manual step pulse. Step is
a pushbutton pulling a 10 k-pulled-up node down, 100 nF across it, into a
Schmitt inverter — about a millisecond of RC, then a hard edge. The switch
common carries a 10 k pull-down so the output stays defined while the slider is
between contacts. A second button pulls the divider's reset high, which zeroes
every stage at once — useful when you want the same phase relationship twice
while probing.
