MPC555
/
MPC556
SYSTEM CONFIGURATION AND PROTECTION
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
6-13
The state of the DEC is not affected by any resets and should be initialized by soft-
ware. The DEC runs continuously after power-up once the time base is enabled by set-
ting the TBE bit of the TBSCR (see
) (unless the clock module is
programmed to turn off the clock). The decrementer continues counting while reset is
asserted.
Loading from the decrementer has no effect on the counter value. Storing to the dec-
rementer replaces the value in the decrementer with the value in the GPR.
Whenever bit zero (the MSB) of the decrementer changes from zero to one, a decre-
menter exception occurs. If software alters the decrementer such that the content of
bit 0 is changed to a value of 1, a decrementer exception occurs.
A decrementer exception causes a decrementer interrupt request to be pending in the
RCPU. When the decrementer exception is taken, the decrementer interrupt request
is automatically cleared.
illustrates some of the periods available for the decrementer, assuming a 4-
MHz or 20-MHz crystal, and TBS = 0 which selects
tbclk
division to FOUR.
NOTE
Time base must be enabled to use the decrementer. See
Time Base Control and Status Register
for more information.
Refer to
3.9.5 Decrementer Register (DEC)
for more information.
6.7 MPC555
/ MPC556 Time Base (TB)
The time base (TB) is a 64-bit free-running binary counter defined by the MPC555 /
MPC556 architecture. The TB has two independent reference registers which can
Table 6-4 Decrementer Time-Out Periods
Count Value
Time-Out @ 4 MHz
Time-Out @ 20 MHz
0
1.0 µs
0.2 µs
9
10 µs
2.0 µs
99
100 µs
20 µs
999
1.0 ms
200 µs
9999
10.0 ms
2 ms
999999
1.0 s
200 ms
9999999
10.0 s
2.0 s
99999999
100.0 s
20 s
999999999
1000. s
200 s
(hex) FFFFFFFF
4295 s
859 s
T
dec
2
32
F
tmbclk
------------------
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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