153
7679H–CAN–08/08
AT90CAN32/64/128
The Timer/Counter Overflow Flag (TOV2) is set each time the counter reaches MAX. If the inter-
rupt 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 OC2A pin.
Setting the COM2A1:0 bits to two will produce a non-inverted PWM and an inverted PWM output
can be generated by setting the COM2A1:0 to three (See
). The actual
OC2A value will only be visible on the port pin if the data direction for the port pin is set as out-
put. The PWM waveform is generated by setting (or clearing) the OC2A Register at the compare
match between OCR2A and TCNT2, and clearing (or setting) the OC2A 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, 32, 64, 128, 256, or 1024).
The extreme values for the OCR2A Register represent special cases when generating a PWM
waveform output in the fast PWM mode. If the OCR2A is set equal to BOTTOM, the output will
be a narrow spike for each MAX+1 timer clock cycle. Setting the OCR2A equal to MAX will result
in a constantly high or low output (depending on the polarity of the output set by the COM2A1:0
bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by set-
ting OC2A to toggle its logical level on each compare match (COM2A1:0 = 1). The waveform
generated will have a maximum frequency of f
oc2A
= f
clk_I/O
/2 when OCR2A is set to zero. This
feature is similar to the OC2A toggle in CTC mode, except the double buffer feature of the Out-
put Compare unit is enabled in the fast PWM mode.
14.7.4
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 repeatedly from BOTTOM to MAX and then from MAX to BOTTOM. In non-
inverting Compare Output mode, the Output Compare (OC2A) is cleared on the compare match
between TCNT2 and OCR2A 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 sym-
metric 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 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
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 OCR2A and TCNT2.
f
OCnxPWM
f
clk_I/O
N
256
⋅
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=
