AT89LP6440 datasheet(78/194 Pages) ATMEL

78 / 194 page

3706A–MICRO–9/09

AT89LP6440 - Preliminary

center-aligned around the timer equal to TOP point. Symmetrical PWM may be used to generate

non-overlapping waveforms.

The TOP value in RCAP2L and RCAP2H is double buffered such that the output frequency is

only updated at the underflow. The channel data register (CCxL, CCxH) is also double-buffered

to prevent glitches. The output frequency and duty cycle for symmetrical PWM are given by the

following equations:

The extreme compare values represent special cases when generating a PWM waveform. If the

compare value is set equal to (or greater than) TOP, the output will remain high or low for non-

inverting and inverting modes, respectively. If the compare value is set to MIN (0000H), the out-

put will remain low or high for non-inverting and inverting modes, respectively.

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.

OUT

Oscillator Frequency

RCAP2H RCAP2L

}

-----------------------------------------------------------------

TPS

---------------------

Non-Inverting:

Duty Cycle

100%

CCxH CCxL

}

RCAP2H RCAP2L

}

------------------------------------------------------

Inverting:

Duty Cycle

100%

RCAP2H RCAP2L

}

CCxH CCxL

}

–

RCAP2H RCAP2L

}

---------------------------------------------------------------------------------------------------

{RCAP2H,RCA2L}

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

(Inverted) CCA

{CCAH,CCAL}

(Non-Inverted) CCB

{CCBH,CCBL}

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