Atmel AVR XMEGA D Series, Manual, Page: 145 / 331

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8210C–AVR–09/11
Atmel AVR XMEGA D
Figure 12-15. Single-slope pulse width modulation.
The PER register defines the PWM resolution. The minimum resolution is 2 bits (PER=0x0003),
and the maximum resolution is 16 bits (PER=MAX).
The following equation calculate the exact resolution for single-slope PWM (R
PWM_SS
):
The single-slope PWM frequency (f
PWM_SS
) depends on the period setting (PER) and the periph-
eral clock frequency (fclk
PER
), and can be calculated by the following equation:
where N represents the prescaler divider used. The waveform generated will have a maximum
frequency of half of the peripheral clock frequency (fclk
PER
) when CCA is set to zero (0x0000)
and no prescaling is used. This also applies when using the hi-res extension, since this
increases the resolution and not the frequency.
12.8.4 Dual-slope PWM
For dual-slope PWM generation, the period (T) is controlled by PER, while CCx registers control
the duty cycle of the WG output.
Figure 12-16 shows how for dual-slope PWM the counter
counts repeatedly from BOTTOM to TOP and then from TOP to BOTTOM. The waveform gener-
ator output is set on BOTTOM, cleared on compare match when up-counting, and set on
compare match when down-counting.
CNT
MAX
TOP
Period (T)
"match"
BOTTOM
WG Output
CCx=BOTTOM
CCx
CCx=TOP
"update"
R
PWM_SS
PER 1
+
( )
log
2
( )
log
----------------------------------- =
f
PWM_SS
fclk
PER
N
PER 1
+
( )
------------------------------- =