PXS20RM Datasheet, PDF (652/1368 Pages) Freescale Semiconductor, Inc – PXS20 Microcontroller

Flexible Motor Control Pulse Width Modulator Module (FlexPWM)
VAL1 ($0100)
VAL3
VAL5
VAL0 ($0000)
VAL4
VAL2
INIT ($FF00)
PWMA
PWMB
Figure 25-3. Center Aligned Example
The submodule timers only count in the up direction and then reset to the INIT value. Instead of having a
single value that determines pulse width, there are two values that must be specified: the turn on edge and
the turn off edge. This double action edge generation not only gives the user control over the pulse width,
but over the relative alignment of the signal as well. As a result, there is no need to support separate PWM
alignment modes since the PWM alignment mode is inherently a function of the turn on and turn off edge
values.
Figure 25-3 also illustrates an additional enhancement to the PWM generation process. When the counter
resets, it is reloaded with a user specified value, which may or may not be zero. If the value chosen
happens to be the 2’s complement of the modulus value, then the PWM generator operates in "signed"
mode. This means that if each PWM’s turn on and turn off edge values are also the same number but only
different in their sign, the "on" portion of the output signal will be centered around a count value of zero.
Therefore, only one PWM value needs to be calculated in software and then this value and its negative are
provided to the submodule as the turn off and turn on edges respectively. This technique will result in a
pulse width that always consists of an odd number of timer counts. If all PWM signal edge calculations
follow this same convention, then the signals will be center aligned with respect 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.
25-6
PXS20 Microcontroller Reference Manual, Rev. 1
Freescale Semiconductor