98
2513L–AVR–03/2013
ATmega162/V
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 39 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.
OCR0 changes its value from MAX, like in
Figure 39. When the OCR0 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 OCR0, 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 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 40 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 40. Timer/Counter Timing Diagram, no Prescaling
Figure 41 shows the same timing data, but with the prescaler enabled.
Figure 41. Timer/Counter Timing Diagram, with Prescaler (f
clk_I/O/8)
Figure 42 shows the setting of OCF0 in all modes except CTC mode.
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)