AT89LP428 datasheet(71/149 Pages) ATMEL

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3654A–MICRO–8/09

AT89LP428/828

Figure 13-9. Non-overlapping Waveforms Using Symmetrical PWM

13.4.2.1

Phase and Frequency Correct PWM

When T2CM

1-0 = 10B, the Symmetrical PWM operates in phase and frequency correct mode. In

this mode the compare value double buffer is only updated when the timer equals MIN (under-

flow). This guarantees that the resulting waveform is always symmetrical around the TOP value

as shown in Figure 13-10 because the up and down count compare values are identical. The

TF2 interrupt flag is only set at underflow.

13.4.2.2

Phase Correct PWM

When T2CM

1-0 =11B, the Symmetrical PWM operates in phase correct mode. In this mode the

compare value double buffer is updated when the timer equals MIN (underflow) and TOP (over-

flow). The resulting waveform may not be completely symmetrical around the TOP value as

shown in Figure 13-11 because the up and down count compare values may not be identical.

However, this allows the pulses to be weighted toward one edge or another. The TF2 interrupt

flag is set at both underflow and overflow.

Figure 13-10. Phase and Frequency Correct Symmetrical (Center-Aligned) PWM

{RCAP2H,RCA2L}

CP/RL2 = 0, T2CM1-0 = 10B, DCEN = 0

(Inverted) CCA

{CCAH,CCAL}

(Non-Inverted) CCB

{CCBH,CCBL}

{RCAP2H,RCA2L}

CP/RL2 = 0, T2CM1-0 = 10B, DCEN = 0

Inverted

{CCxH,CCxL}

Non-Inverted

CCx

Duty Cycle Updated

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