MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
17-60
Freescale Semiconductor
When FORCMA and FORCMB are both set, the output flip-flop is set to the compliment of the EDPOL
bit. This is equivalent to FORCMA having precedence over FORCMB when lead dead time insertion is
selected and FORCMB having precedence over FORCMA when trailing dead time insertion is selected.
Duty cycles from 0% to 100% can be generated by setting appropriate A1 and B1 values relative to the
period of the external time base. Setting A1 = 1 generates a 100% duty cycle waveform. If
A1 > period ÷ 2, where period refers to the selected counter bus period, then a 0% duty cycle is produced.
Assuming EDPOL is one and OPWMCB mode with trailing dead time insertion mode is selected, 100%
duty cycle signals can be generated if B1 occurs at or after the cycle boundary (external counter = 1).
NOTE
A special case occurs when A1 is set to the external counter bus period ÷ 2,
which is the maximum value of the external counter. In this case the output
flip-flop is constantly set to the EDPOL bit value.
Internal channel logic prevents matches from one cycle to propagate to the next cycle. In trailing dead time
insertion mode, a B1 match from cycle (
n
) could eventually cross the cycle boundary and occur in cycle
(n+1). In this case the B1 match is masked out and does not cause the output flip-flop to transition.
Therefore matches in cycle (n+1) are not affected by the late B1 matches from cycle (
n
).
shows a 100% duty cycle output signal generated by setting A1 = 4 and B1 = 3. In this case
the trailing edge is positioned at the boundary of cycle (n+1), which is actually considered to belong to
cycle (n+2) and therefore does not cause the output flip-flip to transition.
Figure 17-49. eMIOS PWMCB Mode Example — 100% Duty Cycle (A1 = 4, B1 = 3)
The output disable input, if enabled, causes the output flip-flop to transition to the compliment of EDPOL.
This allows to the channel output flip-flop to be forced to a safety state. The internal channel matches
continue to occur in this case, thus generating flags. When the output disable is negated, the channel output
flip-flop is again controlled by A1 and B1 matches. This process is synchronous, meaning that the output
channel pin transitions only occur on system clock edges.
0x000001
0x000020
0x000004
A1 Value
A2 Value
B1 Value
B2 Value
0x000004
0x000001
Output Flip-Flop
0x000003
0x000015
0x000003
0x000015
0x000003
Selected
Counter Bus
Time
Write to B2
Time
Cycle n
Cycle n+1
Cycle n+2
Dead-Time
Dead-Time
Dead-Time
Summary of Contents for MPC5553
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