![](http://datasheet.mmic.net.cn/30000/MR80C32-20-D_datasheet_2377239/MR80C32-20-D_80.png)
80
8154B–AVR–07/09
ATmega16A
PWM frequency for the output when using phase correct PWM can be calculated by the follow-
ing equation:
The N variable represents the prescale factor (1, 8, 64, 256, or 1024).
The extreme values for the OCR0 Register represent special cases when generating a PWM
waveform output in the phase correct PWM mode. If the OCR0 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 14-7 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:
OCR0A changes its value from MAX, like in
Figure 14-7. When the OCR0A 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 to the result of an up-
counting Compare Match.
The Timer starts counting from a value higher than the one in OCR0A, and for that reason
misses the Compare Match and hence the OCn change that would have happened on the
way up.
14.8
Timer/Counter Timing Diagrams
The Timer/Counter is a synchronous design and the timer clock (clk
T0) is therefore shown as a
clock enable signal in the following figures. The figures include information on when Interrupt
Flags are set.
Figure 14-8 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 14-8. Timer/Counter Timing Diagram, no Prescaling
Figure 14-9 shows the same timing data, but with the prescaler enabled.
fOCnPCPWM
f
clk_I/O
N 510
------------------
=
clk
Tn
(clk
I/O/1)
TOVn
clk
I/O
TCNTn
MAX - 1
MAX
BOTTOM
BOTTOM + 1