Analog Devices ADSP-BF535 Blackfin, Hardware Reference Manual

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Timer Modes
16-16 ADSP-BF535 Blackfin Processor Hardware Reference

In PWM_OUT, the counter is reloaded at the end of every period
as well as at the end of every pulse. The generated waveform
depends on whether
TIMERx_WIDTH_LO
is updated before or after the
pulse width expires, due to the reload sequence described in “Pulse
Width Modulation Mode (PWM_OUT)” on page 16-13 .
Changing the pulse width while the timer is running is typically accom-
plished by having an interrupt service routine write new values to the
width registers. As seen in
Figure 16-9 , this causes the generation of one
incorrect period if the write to the
TIMERx_WIDTH_LO
register occurs before
the ongoing pulse width expires. This is very likely because the interrupt is
requested at the end of a period.
If an application forbids single misaligned PWM patterns, the procedure
illustrated in
Figure 16-10 can be used. It alters the period value tempo-
rarily and restores the original period value the very next PWM cycle in
order to obtain constant PWM periods.
Figure 16-9. Possible Period Failure Due to Width Update with Timer
Running
W idt h
O ld
Per io d
W ro ng
Pe r iod
Pe r io d
W idt h
O ld
W idt h
N e w
W idt h
N ew
P e rio d -
W idt h O ld
P e r iod -
W idt h N e w
P e riod -
W idth Ne w
W ID TH = W idth N ew