fastjack/attiny-clockgen

Use an ATtiny to generate TOD clock signal for the CIA in a C64/C128

Clock generator

This tiny program generates a 50Hz/60Hz clock signal. This is intended to be an alternative to https://github.com/ytmytm/c128-link232?tab=readme-ov-file#attiny85

Calculations:

CTC Frequency Formula

$$ f = \frac{f_{CPU}}{2 \times \text{prescaler} \times (\text{OCR0A} + 1)} $$

For 50 Hz @ 1 MHz (PB1 HIGH):

$$ \text{OCR0A} = \frac{1 \text{ MHz}}{2 \times 64 \times 50} - 1 = 155 $$

Actual: 50.08 Hz [+0.16%]

For 60 Hz @ 1 MHz (PB1 LOW):

$$ \text{OCR0A} = \frac{1 \text{ MHz}}{2 \times 64 \times 60} -1 = 129 $$

Actual: 60.1 Hz [+0.17%]

Timer/Prescaler values

Parameter	8 MHz Clock	1 MHz Clock
Prescaler	1024	64
OCR0A (50 Hz)	77	155
OCR0A (60 Hz)	64	129
Accuracy	±0.1%	±0.17%

Connections

• PB0 (Pin 5) : Outputs the 50 Hz or 60 Hz clock signal
• PB1 (Pin 6) : Input, selects 50 Hz (HIGH) / 60 Hz (LOW)