Specifications

PWM Waveform Generation With Compare Units and PWM Circuits
6-58
After GP timer 1 (or GP timer 3) is started, the compare registers are rewritten
every PWM period with newly determined compare values to adjust the width
(the duty cycle) of PWM outputs that control the switch-on and off duration of
the power devices. Since the compare registers are shadowed, a new value
can be written to them at any time during a period. For the same reason, new
values can be written to the action and period registers at any time during a
period to change the PWM period or to force changes in PWM output defini-
tion.
6.6.4 Symmetric PWM Waveform Generation
A centered or symmetric PWM signal is characterized by modulated pulses
which are centered with respect to each PWM period. The advantage of a sym-
metric PWM signal over an asymmetric PWM signal is that it has two inactive
zones of the same duration: at the beginning and at the end of each PWM peri-
od. This symmetry has been shown to cause less harmonics than an asym-
metric PWM signal in the phase currents of an AC motor such as induction and
DC brushless motors when sinusoidal modulation is used. Figure 6–23 shows
two examples of symmetric PWM waveforms.
Figure 6–23. Symmetric PWM Waveform Generation With Compare Units and PWM
Circuits (x = 1, 3, or 5)
Timer (PWM)
period 1
Dead time
Timer value
PWM
x
(activelow)
PWM
x+1
(active high)
Compare matches
The generation of a symmetric PWM waveform with a compare unit is similar
to the generation of an asymmetric PWM waveform. The only exception is that
GP timer 1 (or GP timer 3) now needs to be put in continuous up-/down-count-
ing mode.