AT89LP428 datasheet(57/149 Pages) ATMEL

57 / 149 page

3654A–MICRO–8/09

AT89LP428/828

Figure 12-3. Timer 2 Waveform: Auto-reload Mode (DCEN = 0)

12.3.2

Up or Down Counter

Setting DCEN = 1 enables Timer 2 to count up or down, as shown in Figure 12-4. 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.

When T2EX = 1 and T2CM

1-0 = 01B, the timer will overflow at TOP and set the TF2 bit. This

overflow also causes MIN to be reloaded into the timer registers. A logic 0 at T2EX makes Timer

2 count down. The timer underflows when TH2 and TL2 equal MIN. The underflow sets the TF2

bit and causes TOP to be reloaded into the timer registers. The behavior of Count Mode 0 ver-

sus Count Mode 0 when DCEN is enabled is shown in Figure 12-5. EXF2 is not toggle in this

mode.

Figure 12-4. Timer 2 Diagram: Auto-reload Mode (T2CM

1-0 = 00B, DCEN = 1)

Max

Min

Bottom

T2CM1-0 = 00B, DCEN = 0

Max

Min

Top

T2CM1-0 = 01B, DCEN = 0

TF2 Set

÷TPS

(Up Counting Reload Value)

(Down Counting Reload Value)

Overflow

Toggle

Timer 2

Interrupt

Count

Direction

1 = Up

0 = Down

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