AT89 datasheet(7/22 Pages) ATMEL

7 / 22 page

AT89C52

4-67

Figure 2 shows Timer 2 automatically counting up when

DCEN = 0. In this mode, two options are selected by bit

EXEN2 in T2CON. If EXEN2 = 0, Timer 2 counts up to

0FFFFH and then sets the TF2 bit upon overflow. The over-

flow also causes the timer registers to be reloaded with the

16-bit value in RCAP2H and RCAP2L. The values in Timer

in Capture ModeRCAP2H and RCAP2L are preset by soft-

ware. If EXEN2 = 1, a 16-bit reload can be triggered either

by an overflow or by a 1-to-0 transition at external input

T2EX. This transition also sets the EXF2 bit. Both the TF2

and EXF2 bits can generate an interrupt if enabled.

Setting the DCEN bit enables Timer 2 to count up or down,

as shown in Figure 3. In this mode, the T2EX pin controls

the direction of the count. A logic 1 at T2EX makes Timer 2

count up. The timer will overflow at 0FFFFH and set the

TF2 bit. This overflow also causes the 16-bit value in

RCAP2H and RCAP2L to be reloaded into the timer regis-

ters, TH2 and TL2, respectively.

A logic 0 at T2EX makes Timer 2 count down. The timer

underflows when TH2 and TL2 equal the values stored in

RCAP2H and RCAP2L. The underflow sets the TF2 bit and

causes 0FFFFH to be reloaded into the timer registers.

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.

Figure 2. Timer 2 Auto Reload Mode (DCEN = 0)

Table 4. T2MOD—Timer 2 Mode Control Register

OSC

EXF2

TF2

T2EX PIN

T2 PIN

TR2

EXEN2

C/T2 = 0

C/T2 = 1

CONTROL

RELOAD

OVERFLOW

CONTROL

TRANSITION

DETECTOR

TIMER 2

INTERRUPT

÷12

RCAP2L

RCAP2H

TH2

TL2

T2MOD Address = 0C9H

Reset Value = XXXX XX00B

Not Bit Addressable

——————

T2OE

DCEN

Bit

76543210

Symbol

Function

—

Not implemented, reserved for future

T2OE

Timer 2 Output Enable bit.

DCEN

When set, this bit allows Timer 2 to be configured as an up/down counter.

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