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Programmable Interrupt Timers (PIT0–PIT3)
29-6
Freescale Semiconductor
29.3
Functional Description
This section describes the PIT functional operation.
29.3.1
Set-and-Forget Timer Operation
This mode of operation is selected when the RLD bit in the PCSR register is set.
When PIT counter reaches a count of 0x0000, PIF flag is set in PCSR
n
. The value in the modulus register
loads into the counter, and the counter begins decrementing toward 0x0000. If the PCSR
n
[PIE] bit is set,
the PIF flag issues an interrupt request to the CPU.
When the PCSR
n
[OVW] bit is set, the counter can be directly initialized by writing to PMR
n
without
having to wait for the count to reach 0x0000.
Figure 29-5. Counter Reloading from the Modulus Latch
29.3.2
Free-Running Timer Operation
This mode of operation is selected when the PCSR
n
[RLD] bit is clear. In this mode, the counter rolls over
from 0x0000 to 0xFFFF without reloading from the modulus latch and continues to decrement.
When the counter reaches a count of 0x0000, PCSR
n
[PIF] flag is set. If the PCSR
n
[PIE] bit is set, PIF flag
issues an interrupt request to the CPU.
Address: 0xFC08_0004 (PCNTR0)
0xFC08_4004 (PCNTR
1
)
0xFC08_8004 (PCNTR
2
)
0xFC08_C004 (PCNTR
3
)
Access: User read only
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
R
PC
W
Reset
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Figure 29-4. PIT Count Register (PCNTR
n
)
Table 29-5. PCNTR
n
Field Descriptions
Field
Description
15–0
PC
Counter value. Reading this field with two 8-bit reads is not guaranteed coherent. Writing to PCNTR
n
has no effect,
and write cycles are terminated normally.
0x0002
0x0001
0x0000
0x0005
0x0005
PIT Clock
Counter
Modulus
PIF
Содержание MCF54455
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