88
ATmega16M1/32M1/64M1 [DATASHEET]
8209E–AVR–11/2012
Figure 14-7. Phase correct PWM mode, timing diagram.
The Timer/Counter Overflow Flag (TOV0) 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 generating of PWM waveforms on the OC0x pins. Setting
the COM0x1:0 bits to two will produce a non-inverted PWM. An inverted PWM output can be generated by setting
the COM0x1:0 to three: Setting the COM0A0 bits to one allows the OC0A pin to toggle on Compare Matches if the
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 OC0x Register at the compare match between OCR0x and TCNT0 when the
counter increments, and setting (or clearing) the OC0x Register at compare match between OCR0x and TCNT0
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, 64, 256, or 1024).
The extreme values for the OCR0A Register represent special cases when generating a PWM waveform output in
the phase correct PWM mode. If the OCR0A 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 14-7 on page 88 OCnx has a transition from high to low even though there is
no Compare Match. The point of this transition is to guarantee symmetry around BOTTOM. There are two cases
that give a transition without Compare Match.
pin value is the same as the result of a down-counting Compare Match. To ensure symmetry around BOTTOM
the OCnx value at MAX must correspond to the result of an up-counting Compare Match
The timer starts counting from a value higher than the one in OCRnx, and for that reason misses the Compare
Match and hence the OCnx change that would have happened on the way up
TOVn interrupt flag set
OCnx interrupt flag set
1
2
3
TCNTn
Period
OCnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)
OCRnx update
f
OCnxPCPWM
f
clk_I/O
N 510
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