Atmel C51 Datenblatt PDF herunterladen (Seite 13)

AT89S53

4-227

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

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.

Figure 2.

Timer 2 in Auto Reload Mode (DCEN = 0)

Table 9.

T2MOD—Timer 2 Mode Control Register

T2MOD Address = 0C9H Reset Value = XXXX XX00B

Not Bit Addressable

——————T2OEDCEN

Bit

76543210

Symbol Function

— Not implemented, reserved for future use.

T2OE Timer 2 Output Enable bit.

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

1 2 ... 8 9 10 11 12 13 14 15 16 17 18 ... 33 34

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Atmel C51 Datenblatt Seite 13