Periodic Timer Period Calculation - Motorola MC68340 User Manual

4.2.2.6.1 Periodic Timer Period Calculation. The period of the periodic timer can be
calculated using the following equation:
periodic interrupt timer period
Solving the equation using a crystal frequency of 32.768-kHz with the prescaler disabled
gives:
periodic interrupt timer period
periodic interrupt timer period
This gives a range from 122 s, with a PITR value of $01 (00000001 binary), to 31.128
ms, with a PITR value of $FF (11111111 binary).
Solving the equation with the prescaler enabled (PTP=1 in the PITR) gives the following
values:
periodic interrupt timer period
periodic interrupt timer period
This gives a range from 62.5 ms, with a PITR value of $01, to 15.94 s, with a PITR value
of $FF.
For fast calculation of periodic timer period using a 32.768-kHz crystal, the following
equations can be used:
With prescaler disabled:
programmable interrupt timer period
With prescaler enabled:
programmable interrupt timer period
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PITR count value
EXTAL frequency/prescaler value
2 2
PITR count value
32768/1
2 2
PITR count value
8192
PITR count value
32768/512
2 2
PITR count value
16
PITR (122 s)
PITR (62.5 ms)
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