![](http://datasheet.mmic.net.cn/30000/MR80C32-20-D_datasheet_2377239/MR80C32-20-D_126.png)
126
8154B–AVR–07/09
ATmega16A
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 17-7 OCn has a transition from high to l ow even though
there is no Compare Match. The point of this transition is to guarantee symmetry around BOT-
TOM. There are two cases that will give 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 be correspond the 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 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.
clk
Tn
(clk
I/O/1)
TOVn
clk
I/O
TCNTn
MAX - 1
MAX
BOTTOM
BOTTOM + 1