104
8025I–AVR–02/09
ATmega48P/88P/168P/328P
symmetry around BOTTOM the OCnx 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 OCRnx, and for that reason
misses the Compare Match and hence the OCnx change that would have happened on the
way up.
12.8 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 12-8 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 12-8. Timer/Counter Timing Diagram, no Prescaling
Figure 12-9 shows the same timing data, but with the prescaler enabled.
Figure 12-9. Timer/Counter Timing Diagram, with Prescaler (f
clk_I/O
/8)
Figure 12-10 shows the setting of OCF0B in all modes and OCF0A in all modes except CTC
mode and PWM mode, where OCR0A is TOP.
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)
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