91
ATmega165A/PA/325A/PA/3250A/PA/645A/P/6450A/P [DATASHEET]
8285E–AVR–02/2013
rect PWM mode is shown on
Figure 15-7. The TCNT0 value is in the timing diagram shown as a histogram for
illustrating the dual-slope operation. The diagram includes non-inverted and inverted PWM outputs. The small hor-
izontal line marks on the TCNT0 slopes represent compare matches between OCR0A and TCNT0.
Figure 15-7. 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 generation of PWM waveforms on the OC0A pin. Setting the
COM0A1:0 bits to two will produce a non-inverted PWM. An inverted PWM output can be generated by setting the
data direction for the port pin is set as output. The PWM waveform is generated by clearing (or setting) the OC0A
Register at the compare match between OCR0A and TCNT0 when the counter increments, and setting (or clear-
ing) the OC0A Register at compare match between OCR0A and TCNT0 when the counter decrements. The PWM
frequency for the output when using phase correct PWM can be calculated 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 15-7 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 BOTTOM. There are two cases that give a
transition without Compare Match.
OCR0A changes its value from MAX, like in
Figure 15-7. When the OCR0A value is MAX the OCn pin value is the
same as the result of a down-counting Compare Match. To ensure symmetry around BOTTOM the OCn 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 OCR0A, and for that reason misses the Compare
Match and hence the OCn change that would have happened on the way up
TOVn Interrupt Flag Set
OCnx Interrupt Flag Set
1
2
3
TCNTn
Period
OCn
(COMnx1:0 = 2)
(COMnx1:0 = 3)
OCRnx Update
f
OCnxPCPWM
fclk_I/O
N 510
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