Information

System Configuration
MPC8308 PowerQUICC II Pro Processor Reference Manual, Rev. 1
5-52 Freescale Semiconductor
Table 5-52 defines the bit fields of PTEVR.
5.5.6 Functional Description
This section provides a functional description of the PIT unit, including a block diagram and discussion of
operational modes.
5.5.6.1 Periodic Interval Timer Unit
The PIT generates periodic interrupts for use with a real-time operating system or the application software.
It consists of a 32-bit down-counter which is decremented by a clock derived from the CSB clock or from
the RTC_PIT_CLOCK. The 32-bit counter decrements to zero when loaded with a initial value from the
periodic interval timer load register (PTLDR). After the timer reaches zero, PTEVR[PIF] is set and an
interrupt is generated if PTCNR[PIM] = 1. At the next count cycle, the value in the PTLDR[CLDV] is
loaded into the counter and the process repeats. When a new value is loaded into the PTLDR[CLDV], the
PIT is updated, the prescaler counter is reset, and the counter begins counting. Setting of PTEVR[PIF]
generates an interrupt that remains pending until PTEVR[PIF] is cleared. If PTEVR[PIF] is set again
before being cleared, the interrupt remains pending until PTEVR[PIF] is cleared. Any write to the
PTLDR[CLDV] stops the current countdown and the count resumes with the new value in
PTLDR[CLDV]. If PTCNR[CLEN] = 0, the PIT cannot count and retains the old count value. PTCTR
contain the PIT current value. The PIT function can be disabled if needed.
Figure 5-41 shows the functional PIT block diagram.
Figure 5-41. Periodic Interval Timer Functional Block Diagram
5.5.6.2 PIT Operational Modes
The PIT unit can operate in the following modes:
• PIT enable/disable mode:
Table 5-52. PTEVR Bit Settings
Bits Name Description
0–30 — Write reserved, read = 0
31 PIF Periodic interrupt flag bit. It is asserted after the SPMPIT counter counts to zero. This status bit should be
cleared by software.
Clock
PTLDR[CLDV]
PTEVR[PIF]
PERIODIC
PTCNR[CLEN]
Disable
Interrupt
System
Clocking
PTPSR[PRSC]
32-Bit
RTC_PIT
_CLOCK
PTCNR[CLIN]
Prescaler
32-Bit
PTCNR[PIM]
PTCTR[CNTV]
Counter