Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 154
Figure 17-7. 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 WGM2:0 = 7
(See Table 17-4 on page 160). 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 17-7 on page 154 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 17-7 on page 154. 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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