MC68HC908AS32A Datasheet, PDF (148/280 Pages) Freescale Semiconductor, Inc – Microcontrollers

Programmable Interrupt Timer (PIT)
PRST — PIT Reset Bit
Setting this write-only bit resets the PIT counter and the PIT prescaler. Setting PRST has no effect on
any other registers. Counting resumes from $0000. PRST is cleared automatically after the PIT
counter is reset and always reads as 0. Reset clears the PRST bit.
1 = Prescaler and PIT counter cleared
0 = No effect
NOTE
Setting the PSTOP and PRST bits simultaneously stops the PIT counter at
a value of $0000.
PPS[2:0] — Prescaler Select Bits
These read/write bits select one of the seven prescaler outputs as the input to the PIT counter as
Table 12-1 shows. Reset clears the PPS[2:0] bits.
Table 12-1. Prescaler Selection
PPS[2:0]
000
001
010
011
100
101
110
111
PIT Clock Source
Internal bus clock ÷1
Internal bus clock ÷ 2
Internal bus clock ÷ 4
Internal bus clock ÷ 8
Internal bus clock ÷ 16
Internal bus clock ÷ 32
Internal bus clock ÷ 64
Internal bus clock ÷ 64
12.7.2 PIT Counter Registers
The two read-only PIT counter registers contain the high and low bytes of the value in the PIT counter.
Reading the high byte (PCNTH) latches the contents of the low byte (PCNTL) into a buffer. Subsequent
reads of PCNTH do not affect the latched PCNTL value until PCNTL is read. Reset clears the PIT counter
registers. Setting the PIT reset bit (PRST) also clears the PIT counter registers.
NOTE
If you read PCNTH during a break interrupt, be sure to unlatch PCNTL by
reading PCNTL before exiting the break interrupt. Otherwise, PCNTL
retains the value latched during the break.
Address: $004C
Bit 7
6
5
4
3
2
1
Bit 0
Read: Bit 15
14
13
12
11
10
9
Bit 8
Write:
Reset: 0
0
0
0
0
0
0
0
Figure 12-3. PIT Counter Registers (PCNTH–PCNTL)
MC68HC908AS32A Data Sheet, Rev. 2.0
148
Freescale Semiconductor