AT89C52 데이터 시트보기 (PDF) - Atmel Corporation

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AT89C52

8-Bit Microcontroller with 8K Bytes Flash

Atmel Corporation 

AT89C52

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

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

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.

OSC

÷12

T2 PIN

C/T2 = 0

C/T2 = 1

CONTROL

TR2

RELOAD

TH2

TL2

RCAP2H RCAP2L

OVERFLOW

TIMER 2

INTERRUPT

T2EX PIN

TRANSITION

DETECTOR

TF2

EXF2

CONTROL

EXEN2

Table 4. T2MOD—Timer 2 Mode Control Register

T2MOD Address = 0C9H

Reset Value = XXXX XX00B

Not Bit Addressable

—

—

—

—

—

—

T2OE

DCEN

Bit

7

6

5

4

3

2

1

0

Symbol

—

T2OE

DCEN

Function

Not implemented, reserved for future

Timer 2 Output Enable bit.

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

4-67

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