16

1921C–MICRO–3/05

AT89C55WD

Figure 14-1. Timer 2 in Clock-Out Mode

15. Interrupts

The AT89C55WD has a total of six interrupt vectors: two external interrupts (INT0 and INT1),

three timer interrupts (Timers 0, 1, and 2), and the serial port interrupt. These interrupts are all

shown in Figure 15-1.

Each of these interrupt sources can be individually enabled or disabled by setting or clearing a

bit in Special Function Register IE. IE also contains a global disable bit, EA, which disables all

interrupts at once.

Note that Table 5 shows that bit position IE.6 is unimplemented. User software should not write

a ‘1’ to this bit position, since it may be used in future AT89 products.

Timer 2 interrupt is generated by the logical OR of bits TF2 and EXF2 in register T2CON. Nei-

ther of these flags is cleared by hardware when the service routine is vectored to. In fact, the

service routine may have to determine whether it was TF2 or EXF2 that generated the interrupt,

and that bit will have to be cleared in software.

OSC

EXF2

P1.0

(T2)

P1.1

(T2EX)

TR2

EXEN2

C/T2 BIT

TRANSITION

DETECTOR

TIMER 2

INTERRUPT

T2OE (T2MOD.1)

÷2

TL2

(8-BITS)

RCAP2L RCAP2H

TH2

(8-BITS)

÷2