PXS20RM Datasheet, PDF (653/1368 Pages) Freescale Semiconductor, Inc – PXS20 Microcontroller

Flexible Motor Control Pulse Width Modulator Module (FlexPWM)
25.3.2.2 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 25-4. Therefore, only the turn off edge value needs to be periodically updated to
change the pulse width.
VAL1 ($0100)
VAL5
VAL0 ($0000)
VAL3
INIT ($FF00)
PWMA
PWMB
Figure 25-4. 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 zero
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.
25.3.2.3 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 ARE applied to the turn on and turn
off edges of different PWM signal, the signals will be phase shifted with respect to each other, as illustrated
in Figure 25-5. This results in certain advantages when applied to a power stage. For example, when
operating a multi-phase inverter at a low modulation index, all of the PWM switching edges from the
different phases occur at nearly the same time. This can be troublesome from a noise standpoint, especially
Freescale Semiconductor
PXS20 Microcontroller Reference Manual, Rev. 1
25-7