SAM3U_14 Datasheet, PDF (878/1187 Pages) ATMEL Corporation – From 64 to 256 Kbytes embedded Flash

• the waveform polarity. At the beginning of the period, the signal can be at high or low level.
This property is defined in the CPOL field of the PWM_CMRx register. By default the signal
starts by a low level.
• the waveform alignment. The output waveform can be left or center aligned. Center aligned
waveforms can be used to generate non overlapped waveforms. This property is defined in
the CALG field of the PWM_CMRx register. The default mode is left aligned.
Figure 38-4. Non Overlapped Center Aligned Waveforms
No overlap
OC0
OC1
Period
Note: 1. See Figure 38-5 on page 879 for a detailed description of center aligned waveforms.
When center aligned, the channel counter increases up to CPRD and decreases down to 0. This
ends the period.
When left aligned, the channel counter increases up to CPRD and is reset. This ends the period.
Thus, for the same CPRD value, the period for a center aligned channel is twice the period for a
left aligned channel.
Waveforms are fixed at 0 when:
• CDTY = CPRD and CPOL = 0
• CDTY = 0 and CPOL = 1
Waveforms are fixed at 1 (once the channel is enabled) when:
• CDTY = 0 and CPOL = 0
• CDTY = CPRD and CPOL = 1
The waveform polarity must be set before enabling the channel. This immediately affects the
channel output level. Changes on channel polarity are not taken into account while the channel
is enabled.
Besides generating output signals OCx, the comparator generates interrupts in function of the
counter value. When the output waveform is left aligned, the interrupt occurs at the end of the
counter period. When the output waveform is center aligned, the bit CES of the PWM_CMRx
register defines when the channel counter interrupt occurs. If CES is set to 0, the interrupt
occurs at the end of the counter period. If CES is set to 1, the interrupt occurs at the end of the
counter period and at half of the counter period.
Figure 38-5 “Waveform Properties” illustrates the counter interrupts in function of the
configuration.
878 SAM3U Series
6430F–ATARM–21-Feb-12