ATmega64A datasheet(154/392 Pages) ATMEL

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154

8160C–AVR–07/09

ATmega64A

A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by set-

ting OC2 to toggle its logical level on each Compare Match (COM21:0 = 1). The waveform

generated will have a maximum frequency of f

OC2 = fclk_I/O/2 when OCR2 is set to zero. This fea-

ture is similar to the OC2 toggle in CTC mode, except the double buffer feature of the Output

Compare unit is enabled in the fast PWM mode.

17.7.4

Phase Correct PWM Mode

The phase correct PWM mode (WGM21:0 = 1) provides a high resolution phase correct PWM

waveform generation option. The phase correct PWM mode is based on a dual-slope operation.

The counter counts repeatedly from BOTTOM to MAX and then from MAX to BOTTOM. In non-

inverting Compare Output mode, the Output Compare (OC2) is cleared on the Compare Match

between TCNT2 and OCR2 while upcounting, and set on the Compare Match while downcount-

ing. In inverting Output Compare mode, the operation is inverted. The dual-slope operation has

lower maximum operation frequency than single slope operation. However, due to the symmet-

ric feature of the dual-slope PWM modes, these modes are preferred for motor control

applications.

The PWM resolution for the phase correct PWM mode is fixed to eight bits. In phase correct

PWM mode the counter is incremented until the counter value matches MAX. When the counter

reaches MAX, it changes the count direction. The TCNT2 value will be equal to MAX for one

timer clock cycle. The timing diagram for the phase correct PWM mode is shown on Figure 17-7.

The TCNT2 value is in the timing diagram shown as a histogram for illustrating the dual-slope

operation. The diagram includes non-inverted and inverted PWM outputs. The small horizontal

line marks on the TCNT2 slopes represent Compare Matches between OCR2 and TCNT2.

Figure 17-7. Phase Correct PWM Mode, Timing Diagram

TOVn Interrupt

Flag Set

OCn Interrupt

Flag Set

TCNTn

Period

OCn

(COMn1:0 = 2)

(COMn1:0 = 3)

OCRn Update

ATmega64A Datasheet(PDF) 154 Page - ATMEL Corporation