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PIC Microcontrollers DISCUSSION
For an instructional PIC18F452 setup using a 20 MHz oscillator, 16-bit Timer0 and an internal timer clock with a 1:8 prescaler, what preload gives a nominal 10 ms overflow interval?
Why might the measured interval still differ from the simple calculation when the timer is reloaded inside an interrupt?
For that clock selection, the instruction clock is Fosc / 4, or 5 MHz. Dividing by eight gives 625 kHz, so each timer count is 1.6 microseconds. Ten milliseconds requires 6,250 counts. The nominal 16-bit preload is therefore 65,536 - 6,250 = 59,286.
Confirm the actual oscillator configuration and Timer0 mode before using the number. A different clock source, prescaler assignment or eight-bit mode invalidates the calculation. Follow the datasheet's register-access sequence when loading a 16-bit value, and initialize and clear the relevant interrupt state before enabling periodic operation.
The calculation excludes interrupt latency, reload execution and any documented effects of writing the timer. Measure a diagnostic output over many periods to identify jitter and cumulative error. For more precise periodic timing, investigate the device's compare or hardware-period facilities instead of assuming a software reload has zero overhead.
