Datasheet

91
ATmega8A [DATASHEET]
8159E–AVR–02/2013
If OCR1A is used to define the TOP value (WGM13:0 = 9) and COM1A1:0 = 1, the OC1A output will toggle with a
50% duty cycle.
17.10 Timer/Counter Timing Diagrams
The Timer/Counter is a synchronous design and the timer clock (clk
T1
) is therefore shown as a clock enable signal
in the following figures. The figures include information on when Interrupt Flags are set, and when the OCR1x Reg-
ister is updated with the OCR1x buffer value (only for modes utilizing double buffering). Figure 17-10 shows a
timing diagram for the setting of OCF1x.
Figure 17-10. Timer/Counter Timing Diagram, Setting of OCF1x, no Prescaling
Figure 17-11 shows the same timing data, but with the prescaler enabled.
Figure 17-11. Timer/Counter Timing Diagram, Setting of OCF1x, with Prescaler (f
clk_I/O
/8)
Figure 17-12 shows the count sequence close to TOP in various modes. When using phase and frequency correct
PWM mode the OCR1x Register is updated at BOTTOM. The timing diagrams will be the same, but TOP should
be replaced by BOTTOM, TOP-1 by BOTTOM+1 and so on. The same renaming applies for modes that set the
TOV1 Flag at BOTTOM.
clk
Tn
(clk
I/O
/1)
OCFnx
clk
I/O
OCRnx
TCNTn
OCRnx Value
OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2
OCFnx
OCRnx
TCNTn
OCRnx Value
OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2
clk
I/O
clk
Tn
(clk
I/O
/8)