> but what I cannot understand is why a 32768Hz-crystal is magical

As others have said, it's divisible by binary powers, as can be seen
if you look at it in hexadecimal

32768 = $8000

This is good for a clock as a large number of cycles is a whole or
easy fraction of a second when used to clock one of the PIC's timer/
counters. On the F84 you have TMR0. When assigned to count
instruction cycles, it will count 256 (00 to FF) and then rollover to
00, setting T0IF, which can be used as an interrupt flag. A 32768
crystal will cause this rollover 32768/256 = 128 times per second.
If you set the pre-scaler to 1:128, then it now takes 32768 cycles
to set T0IF, or IOW a 1Hz interrupt

Note that the $8000 above can be broken down like this -

The 00 is the low byte, and in the above example is what is counted
by the TMR0 register. The 80 (= d128) is the high byte, and that
is what is counted by the pre-scaler

Another hex one is 9.8304MHz = $960000, which is good for high-
speed RS232 (most baud rates are hexadecimally-based), especially
if used with the 4 x PLL on an 18F

4MHz, 10MHz or 20MHz is better for shorter, exact periods, for
example 100us, because the cycle times are respectively 1us, 400ns
and 200ns. You might use something like this in a timer capture to
measure a fast event (eg ultrasonic sounding) or generate pulses. A
crystal like a 19.6608MHz, good for a clock/RS232, has a cycle
time of 203.45052083ns, which is not a pretty number to work with
if you want exactly 10us, 50us or 100us

So you choose your crystal based on the jobs the micro has to do

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