> What is the relationship between ticks and uC frequency ?

20MHz crystal = 20,000,000 / 4 instruction cycles / sec = 200ns,
ie 5MHz actual processing speed

Note - no TMR1 on the F84. You need an F628

Free-running TMR0 will take 256 * 200ns = 51.2us to rollover

TMR0 pre-scaler (1:2 ......1:256) => 102.4us to 13107.2us

To get an exact 1 second interval, you could change to an
18.432MHz crystal + 1:16 prescaler

18432000/4 = 4608000

4608000/256/16 = 1125 TMR0 rollovers/sec

Any hex value (2^xx * 1000) crystal will divide down easily

eg 3.2768MHz and 1:32 pre-scaler

3276800/4/256/32 = 100 TMR0 rollovers/sec

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