Atmel ATmega128 Manual page 154

ATmega128
154
Figure 67. Phase Correct PWM Mode, Timing Diagram
TCNTn
OCn
OCn
Period
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
OC2 pin. Setting the COM21:0 bits to 2 will produce a non-inverted PWM. An inverted PWM out-
put can be generated by setting the COM21:0 to 3 (see
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 OC2 Register at the compare match
between OCR2 and TCNT2 when the counter increments, and setting (or clearing) the OC2
Register at compare match between OCR2 and TCNT2 when the counter decrements. The
PWM frequency for the output when using phase correct PWM can be calculated by the follow-
ing equation:
The N variable represents the prescale factor (1, 8, 64, 256, or 1024).
The extreme values for the OCR2 Register represent special cases when generating a PWM
waveform output in the phase correct PWM mode. If the OCR2 is set equal to BOTTOM, the out-
put 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
is no Compare Match. The point of this transition is to guarantee symmetry around BOTTOM.
There are two cases that give a transition without a Compare Match:
• OCR2A changes its value from MAX, like in
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.
1 2
f
clk_I/O
f
= ----------------- -
OCnPCPWM
N 510
Figure 67 OCn has a transition from high to low even though there
Figure
OCn Interrupt Flag Set
OCRn Update
TOVn Interrupt Flag Set
(COMn1:0 = 2)
(COMn1:0 = 3)
3
Table 67 on page 158). The actual OC2
⋅
67. When the OCR2A value is MAX the
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