149
2552K–AVR–04/11
ATmega329/3290/649/6490
decrements. The PWM frequency for the output when using phase correct PWM can be calcu-
lated 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 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.
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.
17.8
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
Figure 17-9 shows the same timing data, but with the prescaler enabled.
fOCnxPCPWM
f
clk_I/O
N 510
------------------
=
clk
Tn
(clk
I/O/1)
TOVn
clk
I/O
TCNTn
MAX - 1
MAX
BOTTOM
BOTTOM + 1