AT89LP51RB2 datasheet(92/254 Pages) ATMEL

92 / 254 page

3722A–MICRO–10/11

AT89LP51RB2/RC2/IC2 Preliminary

Figure 14-3. Timer 2 Waveform: Auto-Reload Mode (DCEN = 0)

14.3.2

Up or Down Counter

Setting DCEN = 1 enables Timer 2 to count up or down, as shown in Figure 14-5. In this mode,

the T2EX pin controls the direction of the count (if EXEN2 = 1). A logic 1 at T2EX makes Timer 2

count up. When T2CM

1-0 = 00B, the timer will overflow at MAX and set the TF2 bit. This overflow

also causes BOTTOM, the 16-bit value in RCAP2H and RCAP2L, to be reloaded into the timer

registers, TH2 and TL2, respectively. A logic 0 at T2EX makes Timer 2 count down. The timer

underflows when TH2 and TL2 equal BOTTOM, the 16-bit value stored in RCAP2H and

RCAP2L. The underflow sets the TF2 bit and causes MAX to be reloaded into the timer regis-

ters. The EXF2 bit toggles whenever Timer 2 overflows or underflows and can be used as a 17th

bit of resolution. In this operating mode, EXF2 does not flag an interrupt.

The behavior of Timer 2 when DCEN is enabled is shown in Figure 14-4. The timer over-

flow/underflow rate for up-down counting mode is the same as for up counting mode, provided

that the count direction does not change. Changes to the count direction may result in longer or

shorter periods between time-outs.

Figure 14-4. Timer 2 Waveform: Auto-Reload Mode (DCEN = 1)

MAX

MIN

BOTTOM

TF2 Set

MAX

MIN

BOTTOM

T2EX

TF2 Set

EXF2

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