MC68LC302 Datasheet, PDF (60/169 Pages) Motorola, Inc – Low Power Integrated Multiprotocol Processor Reference Manual

System Integration Block (SIB)
3.7.4.1 Overview
The periodic interrupt timer consists of an 11-bit modulus counter that is loaded with the val-
ue contained in the PITR. The modulus counter is clocked by the CLKIN signal derived from
the IMP EXTAL pin. See Figure 2-2.
The clock source is divided by four before driving the modulus counter (PITCLK). When the
modulus counter value reaches zero, an interrupt request signal is generated to the IMP in-
terrupt controller.
The value of bits 11–1 in the PITR is then loaded again into the modulus counter, and the
counting process starts over. A new value can be written to the PITR only when the PIT is
disabled.
The PIT interrupt replaces the IMP PB8 interrupt and is mapped to the PB8 interrupt priority
level 4. The PIT Interrupt is maskable by setting bit1 (PB8) in the IMR register.
NOTE
When the PIT is enabled, PB8 can still be used as parallel I/O
pin or as DRAM refresh controller request pin, but PB8 will not
be capable of generating interrupts.
3.7.4.2 Periodic Timer Period Calculation
The period of the periodic timer can be calculated using the following equation:
periodic interrupt timer period = ---_(----P---(----I--_E--T------_X^R--------_T----c--_A----o----_{L--u(------4n)------t)--⁄----v--1----a--o----l--r}^{u------5e------+1------21------}_)-----
Solving the equation using a crystal frequency of 32.768 kHz with the prescaler disabled
gives:
periodic interrupt timer period = -P---I--T---R-----3---c---2--o----7--u---2-6--n---₂-8--t-------v_⁄-----1-a----l-u---e---+---1--
periodic interrupt timer period = ^{P----I--T---R------c---o---u---n---t---v---a---l--u---e-}
8192
This gives a range from 122 µs, with a PITR value of $0, to 250 ms, with a PITR value of
$7FF (assuming 32.768 khz at the EXTAL pin.
MOTOROLA
MC68LC302 REFERENCE MANUAL
3-23