inverted PWM output can be generated by setting the COM0x1:0 to three: Setting the
COM0A1:0 bits to one allows the OC0A pin to toggle on Compare Matches if the
WGM02 bit is set. This option is not available for the OC0B pin (See Table 71 on page
128). The actual OC0x 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 OC0x Register at the Compare Match between OCR0x and TCNT0, and clearing (or
setting) the OC0x Register at the timer clock cycle the counter is cleared (changes from
TOP to BOTTOM).
The PWM frequency for the output can be calculated by the following equation:
f
clk_I/O
f
= -----------------
OCnxPWM
N ⋅ 256
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 50ꢀ duty cycle) waveform output in fast PWM mode can be achieved
by setting OC0x to toggle its logical level on each Compare Match (COM0x1:0 = 1). The
waveform generated will have a maximum frequency of fOC0 = 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.
Phase Correct PWM Mode
The phase correct PWM mode (WGM02:0 = 1 or 5) 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 repeatedly from BOTTOM to TOP and then
from TOP to BOTTOM. TOP is defined as 0xFF when WGM2:0 = 1, and OCR0A when
WGM2:0 = 5. In non-inverting Compare Output mode, the Output Compare (OC0x) is
cleared on the Compare Match between TCNT0 and OCR0x while upcounting, and set
on the Compare Match while down-counting. 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.
In phase correct PWM mode the counter is incremented until the counter value matches
TOP. When the counter reaches TOP, it changes the count direction. The TCNT0 value
will be equal to TOP for one timer clock cycle. The timing diagram for the phase correct
PWM mode is shown on Figure 44. The TCNT0 value is in the timing diagram shown as
a histogram for illustrating the dual-slope operation. The diagram includes non-inverted
and inverted PWM outputs. The small horizontal line marks on the TCNT0 slopes repre-
sent Compare Matches between OCR0x and TCNT0.
124
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATMEL [ ATMEL ]
