AT89LP428 datasheet(69/149 Pages) ATMEL

69 / 149 page

3654A–MICRO–8/09

AT89LP428/828

and precision is made by changing the TOP value of the timer. The CCA PWM always uses the

greatest precision allowable for the selected output frequency, as compared to Timer 0 and 1

whose PWMs are fixed at 8-bit precision regardless of frequency.

Figure 13-7. CCA PWM Mode Diagram

13.4.1

Asymmetrical PWM

For Asymmetrical PWM, Timer 2 should be configured for Auto-reload mode and Count Mode 1

(CP/RL2 =0, DCEN = 0, T2CM1-0 = 01B). Asymmetrical PWM uses single slope operation as

shown in Figure 13-8. The timer counts up from BOTTOM to TOP and then restarts from BOT-

TOM. In non-inverting mode, the output CCx is set on the compare match between Timer 2

(TL2, TH2) and the channel data register (CCxL, CCxH), and cleared at BOTTOM. In inverting

mode, the output CCx is cleared on the compare match between Timer 2 and the data register,

and set at BOTTOM. The resulting asymmetrical output waveform is left-edge aligned.

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

only updated at the TOP to BOTTOM overflow. The channel data register (CCxL, CCxH) is also

double-buffered such that the duty cycle is only updated at the TOP to BOTTOM overflow to pre-

vent glitches. The output frequency and duty cycle for asymmetrical 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 low or high for non-

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

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

TL2

TH2

CCxL

CCxH

CCCx

Interrupt

CCx (P2.x)

T2CCC

T2CCL

Shadow

CCFx

CIENx

CxM2-0

T2CCH

Shadow

OUT

Oscillator Frequency

RCAP2H RCAP2L

}

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

TPS

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

Inverting:

Duty Cycle

100%

CCxH CCxL

}

RCAP2H RCAP2L

}

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

Non-Inverting:

Duty Cycle

100%

RCAP2H RCAP2L

}

CCxH CCxL

}

–

RCAP2H RCAP2L

}

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

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