11

2487C–MICRO–03/05

AT89S51

10. Interrupts

The AT89S51 has a total of five interrupt vectors: two external interrupts (INT0 and INT1), two

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

Figure 10-1.

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 10-1 shows that bit positions IE.6 and IE.5 are unimplemented. User software

should not write 1s to these bit positions, since they may be used in future AT89 products.

The Timer 0 and Timer 1 flags, TF0 and TF1, are set at S5P2 of the cycle in which the timers

overflow. The values are then polled by the circuitry in the next cycle.

Table 10-1.

Interrupt Enable (IE) Register

(MSB)

(LSB)

EA

–

–

ES

ET1

EX1

ET0

EX0

Enable Bit = 1 enables the interrupt.

Enable Bit = 0 disables the interrupt.

Symbol

Position

Function

EA

IE.7

Disables all interrupts. If EA = 0, no interrupt is

acknowledged. If EA = 1, each interrupt source is

individually enabled or disabled by setting or clearing its

enable bit.

–

IE.6

Reserved

–

IE.5

Reserved

ES

IE.4

Serial Port interrupt enable bit

ET1

IE.3

Timer 1 interrupt enable bit

EX1

IE.2

External interrupt 1 enable bit

ET0

IE.1

Timer 0 interrupt enable bit

EX0

IE.0

External interrupt 0 enable bit

User software should never write 1s to reserved bits, because they may be used in future AT89

products.