134
ATmega165A/PA/325A/PA/3250A/PA/645A/P/6450A/P [DATASHEET]
8285E–AVR–02/2013
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 18-7 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 BOTTOM. There are two cases that give a
transition without Compare Match.
OCR2A changes its value from MAX, like in
Figure 18-7. 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.
18.8
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 18-8 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 18-8. Timer/Counter Timing Diagram, no Prescaling.
Figure 18-9 shows the same timing data, but with the prescaler enabled.
Figure 18-9. Timer/Counter Timing Diagram, with Prescaler (f
clk_I/O/8).
clk
Tn
(clk
I/O/1)
TOVn
clk
I/O
TCNTn
MAX - 1
MAX
BOTTOM
BOTTOM + 1
TOVn
TCNTn
MAX - 1
MAX
BOTTOM
BOTTOM + 1
clk
I/O
clk
Tn
(clk
I/O/8)