MPC555
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MPC556
MODULAR INPUT/OUTPUT SUBSYSTEM (MIOS1)
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
15-14
as a time reference for other submodules. Note that the MMCSM can also operate as
a free running counter by loading the modulus value of zero.
The main components of the MMCSM are an 8-bit prescaler counter, an 8-bit prescal-
er register, a 16-bit up-counter register, a 16-bit modulus latch register, counter load-
ing and interrupt flag generation logic.
The contents of the modulus latch register is transferred to the counter under the fol-
lowing three conditions:
1. When an overflow occurs
2. When an appropriate transition occurs on the external load pin
3. When the program writes to the counter register. In this case, the value is first
written into the modulus register and immediately transferred to the counter.
Software can also write a value to the modulus register for later loading into the
counter with one of the two first criteria.
A software control register selects whether the clock input to the counter is the pres-
caler output or the corresponding input pin. The polarity of the external input pin is also
programmable.
Refer to
for the MMCSM relative I/O pin implementation.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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