AT89S53(2000) データシートの表示（PDF） - Atmel Corporation

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AT89S53
(Rev.:2000)

8-bit Microcontroller with 12K Bytes Flash

Atmel Corporation 

Capture Mode

In the capture mode, two options are selected by bit

EXEN2 in T2CON. If EXEN2 = 0, Timer 2 is a 16-bit timer

or counter which upon overflow sets bit TF2 in T2CON.

This bit can then be used to generate an interrupt. If

EXEN2 = 1, Timer 2 performs the same operation, but a l-

to-0 transition at external input T2EX also causes the

current value in TH2 and TL2 to be captured into RCAP2H

and RCAP2L, respectively. In addition, the transition at

T2EX causes bit EXF2 in T2CON to be set. The EXF2 bit,

like TF2, can generate an interrupt. The capture mode is

illustrated in Figure 1.

Figure 1. Timer 2 in Capture Mode

OSC

T2 PIN

T2EX PIN

÷12

C/T2 = 0

C/T2 = 1

TRANSITION

DETECTOR

CONTROL

TR2

CAPTURE

CONTROL

EXEN2

TH2

TL2

TF2

OVERFLOW

RCAP2H RCAP2L

EXF2

TIMER 2

INTERRUPT

Auto-reload (Up or Down Counter)

Timer 2 can be programmed to count up or down when

configured in its 16-bit auto-reload mode. This feature is

invoked by the DCEN (Down Counter Enable) bit located in

the SFR T2MOD (see Table 9). Upon reset, the DCEN bit

is set to 0 so that timer 2 will default to count up. When

DCEN is set, Timer 2 can count up or down, depending on

the value of the T2EX pin.

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

overflow also causes the timer registers to be reloaded with

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

RCAP2H and RCAP2L are preset by software. 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.

10

AT89S53

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