90
ATmega165A/PA/325A/PA/3250A/PA/645A/P/6450A/P [DATASHEET]
8285E–AVR–02/2013
Figure 15-6. Fast PWM Mode, timing diagram.
The Timer/Counter Overflow Flag (TOV0) 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 OC0A pin. Setting the
COM0A1:0 bits to two will produce a non-inverted PWM and 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 setting (or clearing) the
OC0A Register at the compare match between OCR0A and TCNT0, and clearing (or setting) the OC0A 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:
The N variable represents the prescale factor (1, 8, 64, 256, or 1024).
The extreme values for the OCR0A Register represents special cases when generating a PWM waveform output in
the fast PWM mode. If the OCR0A is set equal to BOTTOM, the output will be a narrow spike for each MAX+1
timer clock cycle. Setting the OCR0A equal to MAX will result in a constantly high or low output (depending on the
polarity of the output set by the COM0A1:0 bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting OC0A to toggle
its logical level on each compare match (COM0A1:0 = 1). The waveform generated will have a maximum fre-
quency of f
OC0 = fclk_I/O/2 when OCR0A is set to zero. This feature is similar to the OC0A toggle in CTC mode,
except the double buffer feature of the Output Compare unit is enabled in the fast PWM mode.
15.7.4
Phase Correct PWM mode
The phase correct PWM mode (WGM01:0 = 1) provides a high resolution phase correct PWM waveform genera-
tion option. The phase correct PWM mode is based on a dual-slope operation. The counter counts repeatedly from
BOTTOM to MAX and then from MAX to BOTTOM. In non-inverting Compare Output mode, the Output Compare
(OC0A) is cleared on the compare match between TCNT0 and OCR0A while upcounting, and set on the compare
match while downcounting. In inverting Output Compare mode, the operation is inverted. The dual-slope operation
has lower maximum operation frequency than single slope operation. However, due to the symmetric feature of the
dual-slope PWM modes, these modes are preferred for motor control applications.
The PWM resolution for the phase correct PWM mode is fixed to eight bits. In phase correct PWM mode the coun-
ter is incremented until the counter value matches MAX. When the counter reaches MAX, it changes the count
direction. The TCNT0 value will be equal to MAX for one timer clock cycle. The timing diagram for the phase cor-
TCNTn
OCRnx Update and
TOVn Interrupt Flag Set
1
Period
2
3
OCn
(COMnx1:0 = 2)
(COMnx1:0 = 3)
OCRnx Interrupt Flag Set
4
5
6
7
f
OCnxPWM
f
clk_I/O
N 256
------------------
=