Datasheet

108
ATmega8A [DATASHEET]
8159E–AVR–02/2013
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)
Figure 18-10 shows the setting of OCF2 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)