MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
17-42
Freescale Semiconductor
In this mode, the duty cycle is (register A1 + 1) and the period is (register B1 + 1). The MODE[6] bit
controls the transfer from register B2 to B1, which can be done either immediately (MODE[6] cleared),
providing the fastest change in the duty cycle, or at every match of register A1 (MODE[6] set).
The internal counter is automatically selected as a time base, therefore the BSL[0:1] bits in register
EMIOS_CCR
n
have no meaning. The output flip-flop’s active state is the complement of EDPOL bit. The
output flip-flop is active during the duty cycle (from the start of the cycle until a match occurs in
comparator A). After the match in comparator A the output flip-flop is in the inactive state (the value of
EDPOL) until the next cycle starts. When a match on comparator A occurs, the output flip-flop is set to
the value of the EDPOL bit. When a match occurs on comparator B, the output flip-flop is set to the
complement of the EDPOL bit and the internal counter is cleared.
FLAG can be generated at match B, when MODE[5] is cleared, or in both matches, when MODE[5] is set.
At any time, the FORCMA and FORCMB bits allow the software to force the output flip-flop to the level
corresponding to a match on A or B respectively. Also, FORCMB clears the internal counter. Note that the
FLAG bit is not set by the FORCMA or FORCMB operations.
If subsequent comparisons occur on comparators A and B, the PWFM pulses continue to be output,
regardless of the state of the FLAG bit.
In order to achieve 0% duty cycle, both registers A1 and B1 must be set to the same value. When a
simultaneous match occurs on comparators A and B, the output flip-flop is set at every period to the value
of EDPOL bit.
To temporarily change from the current duty cycle to 0% and then return to the current duty cycle, the
sequence is the following:
1. If not currently stored, store value of register A.
2. Set A=B.
3. If immediate 0% duty cycle is desired, set FORCA=1.
4. To return to the previous duty cycle, restore register A with its former value.
100% duty cycle is possible by writing 0x000000 to register A. When a match occurs, the output flip-flop
is set at every period to the complement of EDPOL bit. The transfer from register B2 to B1 is still
controlled by MODE[6] bit.
To temporarily change from the current duty cycle to 100% and then return to the current duty cycle, the
sequence is the following:
1. If not currently stored, store value of register A.
2. Set A=0.
3. If immediate 100% duty cycle is desired, set FORCB=1.
4. To return to the previous duty cycle, restore register A with its former value.
NOTE
Updates to the A register will always occur immediately. If next period
update is selected via the mode[6] bit, only the B register update is delayed
until the next period.
shows the unified channel running in OPFWM mode with immediate register update and
shows the unified channel running in OPFWM mode with next period update PFWM mode.
In both figures EDPOL = 1, so the output is low during the duty cycle.
illustrative examples.
Summary of Contents for MPC5553
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