AT89S52-24AC Просмотр технического описания (PDF) - Atmel Corporation

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AT89S52-24AC

8-bit Microcontroller with 8K Bytes In-System Programmable Flash

Atmel Corporation 

AT89S52

Figure 10-1. Timer 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

Table 10-2. 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

Figure 10-2 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 Timer in Capture ModeRCAP2H 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 10-2. 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 registers, 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.

13

1919C–MICRO–3/05

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