User Manual

150
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
8271E–AVR–07/2012
Figure 18-7. 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 OC2x pin. Setting the
COM2x1:0 bits to two will produce a non-inverted PWM. An inverted PWM output can be generated by setting the
COM2x1:0 to three. TOP is defined as 0xFF when WGM2:0 = 3, and OCR2A when MGM2:0 = 7 (See Table 18-4
on page 155). The actual OC2x 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 OC2x Register at the compare match between
OCR2x and TCNT2 when the counter increments, and setting (or clearing) the OC2x Register at compare match
between OCR2x and TCNT2 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, 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 18-7 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.
• OCR2A changes its value from MAX, like in Figure 18-7. When the OCR2A value is MAX the OCn pin value is the
same as the result of a down-counting compare match. To ensure 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 OCR2A, 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
OCnx
OCnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)
OCRnx Update
f
OCnxPCPWM
f
clk_I/O
N 510
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