The extreme values for the OCR1x Register represents special cases when generating a PWM
waveform output in the phase and frequency correct PWM mode. If the OCR1x is set equal to
BOTTOM the output will be continuously low and if set equal to TOP the output will be set to
high for non-inverted PWM mode. For inverted PWM the output will have the opposite logic val-
ues. If OCR1A is used to define the TOP value (WGM13:0 = 9) and COM1A1:0 = 1, the OC1A
output will toggle with a 50ꢀ duty cycle.
14.9 Timer/Counter Timing Diagrams
The Timer/Counter is a synchronous design and the timer clock (clkT1) 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 Register is updated with the OCR1x buffer value (only for
modes utilizing double buffering). Figure 14-10 shows a timing diagram for the setting of OCF1x.
Figure 14-10. Timer/Counter Timing Diagram, Setting of OCF1x, no Prescaling
clkI/O
clkTn
(clkI/O/1)
TCNTn
OCRnx
OCFnx
OCRnx - 1
OCRnx
OCRnx + 1
OCRnx + 2
OCRnx Value
Figure 14-11 shows the same timing data, but with the prescaler enabled.
Figure 14-11. Timer/Counter Timing Diagram, Setting of OCF1x, with Prescaler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O/8)
TCNTn
OCRnx
OCFnx
OCRnx - 1
OCRnx
OCRnx + 1
OCRnx + 2
OCRnx Value
Figure 14-12 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
126
ATmega169P
8018A–AVR–03/06
ATMEL [ ATMEL ]
