MC68LC302 Datasheet, PDF (61/169 Pages) Motorola, Inc – Low Power Integrated Multiprotocol Processor Reference Manual

System Integration Block (SIB)
Solving the equation with the prescaler enabled (PTP=1) gives the following values:
periodic interrupt timer period = -P---I--T---3--R---2------7---c---6--o----8--u------n-_⁄-----t5------1-v-----2-a----l--u---e-
2²
periodic interrupt timer period = ^{P----I--T---R------c---o-1--u-6--n---t---v---a---l--u---e-}
This gives a range from 62.5 ms, with a PITR value of $0 to 128 s, with a PITR value of $7FF.
For a fast calculation of periodic timer period using a 32.768-kHz crystal, the following equa-
tions can be used:
With prescaler disabled:
programmable interrupt timer period = PITR (122 µs)
With prescaler enabled:
programmable interrupt timer period = PITR (62.5 ms)
3.7.4.3 Using the Periodic Timer As a Real-Time Clock
The periodic interrupt timer can be used as a real-time clock interrupt by setting it up to gen-
erate an interrupt with a one-second period. When using a 32.768-kHz crystal, the PITR
should be loaded with a value of $0F with the prescaler enabled to generate interrupts at a
one-second rate. The interrupt is generated, in this case, at a precise 1 second rate, even if
the interrupt is not serviced immediately. A true real time clock is obtained if the current in-
terrupt is serviced completely before the next one occurs.
3.7.4.4 Periodic Interrupt Timer Register (PITR)
The PITR contains control for prescaling the periodic timer as well as the count value for the
periodic timer. This register can be read or written only during normal operational mode. Bits
14–13 are not implemented and always return a zero when read. A write does not affect
these bits.
PITR
15
14
PTEN
0
RESET
0
0
$0F0
13
12
11
10
9
8
0
PTP
PITR10
PITR9
PITR8
PITR7
0
0
0
0
0
0
7
6
5
4
3
2
1
0
PITR6
PITR5
PITR4
PITR3
PITR2
PITR1
PITR0
RES
RESET
0
0
0
0
0
0
0
0
Read/Write
3-24
MC68LC302 REFERENCE MANUAL
MOTOROLA