FlexPWM RM0046
676/936 Doc ID 16912 Rev 5
to each other, which is the goal. Of course, center alignment between the signals is not
restricted to symmetry around the zero count value, as any other number would also work.
However, centering on zero provides the greatest range in signed mode and also simplifies
the calculations.
25.7.2 Edge-aligned PWMs
When the turn on edge for each pulse is specified to be the INIT value, then edge-aligned
operation results, as illustrated in Figure 362. Therefore, only the turn off edge value needs
to be periodically updated to change the pulse width.
Figure 362. Edge-aligned example (INIT = VAL2 = VAL4)
With edge-aligned PWMs, another example of the benefits of signed mode can be seen. A
common way to drive an H-bridge is to use a technique called “bipolar” PWMs where a 50%
duty cycle results in 0 volts on the load. Duty cycles less than 50% result in negative load
voltages and duty cycles greater than 50% generate positive load voltages. If the module is
set to signed mode operation (the INIT and VAL1 values are the same number with opposite
signs), then there is a direct proportionality between the PWM turn off edge value and the
motor voltage, INCLUDING the sign. So once again, signed mode of operation simplifies the
software interface to the PWM module since no offset calculations are required to translate
the output variable control algorithm to the voltage on an H-Bridge load.
25.7.3 Phase-shifted PWMs
In the previous sections, the benefits of signed mode of operation were discussed in the
context of simplifying the required software calculations by eliminating the requirement to
bias up signed variables before applying them to the module. However, if numerical biases
VAL1 (0x0100)
VAL5
VAL0 (0x0000)
VAL3
INIT (0xFF00)
PWMA
PWMB
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