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2513L–AVR–03/2013
ATmega162/V
actual OC2 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 OC2 Register at the Com-
pare Match between OCR2 and TCNT2 when the counter increments, and setting (or clearing)
the OC2 Register at Compare Match between OCR2 and TCNT2 when the counter decrements.
The PWM frequency for the output when using phase correct PWM can be calculated by the fol-
lowing equation:
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 phase correct PWM mode. If the OCR2 is set equal to BOTTOM, the out-
put 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 65 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 BOTTOM.
There are two cases that give a transition without a Compare Match.
OCR2 changes its value from MAX, like in
Figure 65. When the OCR2 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 OCR2, 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 66 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 66. Timer/Counter Timing Diagram, no Prescaling
Figure 67 shows the same timing data, but with the prescaler enabled.
f
OCnPCPWM
fclk_I/O
N 510
------------------
=
clk
Tn
(clk
I/O/1)
TOVn
clk
I/O
TCNTn
MAX - 1
MAX
BOTTOM
BOTTOM + 1