Datasheet

small horizontal line marks on the TCNT2 slopes represent compare matches between OCR2 and
TCNT2.
Figure 22-6. Fast PWM Mode, Timing Diagram
TCNTn
OCRn Update
and
TOVn Interrupt Flag Set
1
Period
2 3
OCn
OCn
(COMn1:0 = 2)
(COMn1:0 = 3)
OCRn Interrupt Flag Set
4 5 6 7
The Timer/Counter Overflow Flag (TOV2) is set each time the counter reaches MAX. If the interrupt is
enabled, the interrupt handler routine can be used for updating the compare value.
In fast 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 and an inverted PWM output can be generated by
setting the COM21:0 to 3. The actual OC2 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 setting (or clearing) the OC2 Register at
the Compare Match between OCR2 and TCNT2, and clearing (or setting) the OC2 Register at the timer
clock cycle the counter is cleared (changes from MAX to BOTTOM).
The PWM frequency for the output can be calculated by the following equation:

OCnPWM
=

clk_I/O
  256
The N variable represents the prescaler factor (1, 8, 32, 64, 128, 256, or 1024).
The extreme values for the OCR2 Register represent special cases when generating a PWM waveform
output in the fast PWM mode. If the OCR2 is set equal to BOTTOM, the output will be a narrow spike for
each MAX+1 timer clock cycle. Setting the OCR2 equal to MAX will result in a constantly high or low
output (depending on the polarity of the output set by the COM21:0 bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting OC2
to toggle its logical level on each Compare Match (COM21:0 = 1). The waveform generated will have a
maximum frequency of f
oc2
= f
clk_I/O
/2 when OCR2 is set to zero. This feature is similar to the OC2 toggle
in CTC mode, except the double buffer feature of the Output Compare unit is enabled in the fast PWM
mode.
22.7.4 Phase Correct PWM Mode
The phase correct PWM mode (WGM21:0 = 1) provides a high resolution phase correct PWM waveform
generation option. The phase correct PWM mode is based on a dual-slope operation. The counter counts
AVR 8-Bit Microcontroller
8-bit Timer/Counter2 with PWM and Asynchronous Ope...
© 2017 Microchip Technology Inc.
Datasheet Complete
40001974A-page 157