ATmega128 datasheet(131/386 Pages) ATMEL

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131

2467S–AVR–07/09

ATmega128

The extreme values for the OCRnx Register represent special cases when generating a PWM

waveform output in the phase correct PWM mode. If the OCRnx 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 values.

If OCRnA is used to define the TOP value (WGMn3:0 = 9) and COMnA1:0 = 1, the OCnA Output

will toggle with a 50% duty cycle.

Timer/Counter

Timing Diagrams

The Timer/Counter is a synchronous design and the timer clock (clk

Tn) 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 OCRnx Register is updated with the OCRnx buffer value (only for

modes utilizing double buffering). Figure 55 shows a timing diagram for the setting of OCFnx.

Figure 55. Timer/Counter Timing Diagram, Setting of OCFnx, no Prescaling

Figure 56 shows the same timing data, but with the prescaler enabled.

Figure 56. Timer/Counter Timing Diagram, Setting of OCFnx, with Prescaler (f

clk_I/O/8)

clk

(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

(clk

I/O/8)

ATmega128 Datasheet(PDF) 131 Page - ATMEL Corporation

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