ATMEGA329P_14 Datasheet, PDF (150/427 Pages) ATMEL Corporation – High Performance, Low Power Atmel

ATmega329P/3290P
In phase correct 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. An inverted
PWM output can be generated by setting the COM2A1:0 to three (See Table 17-5 on page 156).
The actual OC2A 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 clearing (or setting) the OC2A Register at the
compare match between OCR2A and TCNT2 when the counter increments, and setting (or
clearing) the OC2A Register at compare match between OCR2A and TCNT2 when the counter
decrements. The PWM frequency for the output when using phase correct PWM can be calcu-
lated by the following equation:
f_OCnxPCPWM
=
-^f--^c---^l-^k--^_---^I-^/-^O---
N ⋅ 510
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 phase correct PWM mode. If the OCR2A is set equal to BOTTOM, the
output will be continuously low and if set equal to MAX the output will be continuously high for
non-inverted PWM mode. For inverted PWM the output will have the opposite logic values.
At the very start of period 2 in Figure 17-7 OCn has a transition from high to low even though
there is no Compare Match. The point of this transition is to guarantee symmetry around BOT-
TOM. There are two cases that give a transition without Compare Match.
17.8
• OCR2A changes its value from MAX, like in Figure 17-7. When the OCR2A value is MAX the
OCn pin value is the same as the result of a down-counting compare match. To ensure
symmetry around BOTTOM the OCn value at MAX must correspond to the result of an up-
counting Compare Match.
• The timer starts counting from a value higher than the one in OCR2A, and for that reason
misses the Compare Match and hence the OCn change that would have happened on the way
up.
Timer/Counter Timing Diagrams
The following figures show the Timer/Counter in synchronous mode, and the timer clock (clk_T2)
is therefore shown as a clock enable signal. In asynchronous mode, clk_I/O should be replaced by
the Timer/Counter Oscillator clock. The figures include information on when Interrupt Flags are
set. Figure 17-8 contains timing data for basic Timer/Counter operation. The figure shows the
count sequence close to the MAX value in all modes other than phase correct PWM mode.
Figure 17-8. Timer/Counter Timing Diagram, no Prescaling
clk_I/O
clk_Tn
(clk_I/O/1)
TCNTn
MAX - 1
MAX
BOTTOM
BOTTOM + 1
TOVn
8021G–AVR–03/11
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