144
8023F–AVR–07/09
ATmega325P/3250P
Figure 16-7. Phase Correct PWM Mode, Timing Diagram
The Timer/Counter Overflow Flag (TOV2) is set each time the counter reaches BOTTOM. The
Interrupt Flag can be used to generate an interrupt each time the counter reaches the BOTTOM
value.
In phase correct PWM mode, the compare unit allows generation of PWM waveforms on the
OC2A pin. Setting the COM2A1:0 bits to two will produce a non-inverted PWM. An inverted
The actual OC2A value will only be visible on the port pin if the data direction for the port pin is
set as output. The PWM waveform is generated by clearing (or setting) the OC2A Register at the
compare match between OCR2A and TCNT2 when the counter increments, and setting (or
clearing) the OC2A Register at compare match between OCR2A and TCNT2 when the counter
decrements. The PWM frequency for the output when using phase correct PWM can be calcu-
lated by the following equation:
The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).
The extreme values for the OCR2A Register represent special cases when generating a PWM
waveform output in the phase correct PWM mode. If the OCR2A is set equal to BOTTOM, the
output will be continuously low and if set equal to MAX the output will be continuously high for
non-inverted PWM mode. For inverted PWM the output will have the opposite logic values.
At the very start of period 2 in
Figure 16-7 OCn has a transition from high to low even though
there is no Compare Match. The point of this transition is to guarantee symmetry around BOT-
TOM. There are two cases that give a transition without Compare Match.
OCR2A changes its value from MAX, like in
Figure 16-7. When the OCR2A value is MAX the
OCn pin value is the same as the result of a down-counting compare match. To ensure
TOVn Interrupt Flag Set
OCnx Interrupt Flag Set
1
2
3
TCNTn
Period
OCnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)
OCRnx Update
fOCnxPCPWM
f
clk_I/O
N 510
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