The PWM frequency for the output can be calculated by the following equation:
fclk_I/O
fOCnPWM = -----------------
N ⋅ 256
The N variable represents the prescale 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 out-
put 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 foc2 = fclk_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.
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 repeatedly from BOTTOM to MAX and then from
MAX to BOTTOM. In non-inverting Compare Output mode, the Output Compare (OC2)
is cleared on the compare match between TCNT2 and OCR2 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 8 bits. In phase correct
PWM mode the counter is incremented until the counter value matches MAX. When the
counter reaches MAX, it changes the count direction. The TCNT2 value will be equal to
MAX for one timer clock cycle. The timing diagram for the phase correct PWM mode is
shown on Figure 59. The TCNT2 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 TCNT2 slopes represent compare
matches between OCR2 and TCNT2.
118
ATmega16(L)
2466E–AVR–10/02
ATMEL [ ATMEL ]
