Interrupt is an external or internal event that disturb the microcontroller to inform it that a device needs its service, the program which is associated with the interrupt is called interrupt service routine (ISR) or interrupt handler.

8051 microcontroller has five interrupt:

1. Timer 0 overflow interrupt
2. Timer 1 overflow interrupt
3. External interrupt 0 (INT 0)
4. External interrupt 1 (INT 1)
5. Serial port events (buffer full, buffer empty)

Each interrupt has specific place in code memory where program execution (interrupt service routine) begin

• External interrupt 0 : 00034
• Timer overflow : 000BH
• External interrupt 1 : 00BH
• Timer 1 overflow : 001BH
• Serial interrupt : 00234

These interrupts must be enabled by software in order for the microcontroller to respond to them. This is done by 8-bit register called Interrupt Enable Register (IER)

7-bit : EA : Global enable/disable, to enable the interrupt this bit must be set high

6-bit : - : Undefined reserved for future use

5-bit : ET2 : Enable/disable timer 2 overflow interrupt

4-bit : ES : Enable/disable serial port interrupt

3-bit : ET1 : Enable/disable Timer 1 overflow interrupt

2-bit : EX1 : Enable/disable external interrupt 1

1-bit : ET0 : Enable/disable timer 0 overflow interrupt

0-bit : EX0 : Enable/disable external interrupt 0