Microcontrollers - 8051 Interrupts


Interrupts are the events that temporarily suspend the main program, pass the control to the external sources and execute their task. It then passes the control to the main program where it had left off.

8051 has 5 interrupt signals, i.e. INT0, TFO, INT1, TF1, RI/TI. Each interrupt can be enabled or disabled by setting bits of the IE register and the whole interrupt system can be disabled by clearing the EA bit of the same register.

IE (Interrupt Enable) Register

This register is responsible for enabling and disabling the interrupt. EA register is set to one for enabling interrupts and set to 0 for disabling the interrupts. Its bit sequence and their meanings are shown in the following figure.

IE Register

EA IE.7 It disables all interrupts. When EA = 0 no interrupt will be acknowledged and EA = 1 enables the interrupt individually.
- IE.6 Reserved for future use.
- IE.5 Reserved for future use.
ES IE.4 Enables/disables serial port interrupt.
ET1 IE.3 Enables/disables timer1 overflow interrupt.
EX1 IE.2 Enables/disables external interrupt1.
ET0 IE.1 Enables/disables timer0 overflow interrupt.
EX0 IE.0 Enables/disables external interrupt0.

IP (Interrupt Priority) Register

We can change the priority levels of the interrupts by changing the corresponding bit in the Interrupt Priority (IP) register as shown in the following figure.

  • A low priority interrupt can only be interrupted by the high priority interrupt, but not interrupted by another low priority interrupt.

  • If two interrupts of different priority levels are received simultaneously, the request of higher priority level is served.

  • If the requests of the same priority levels are received simultaneously, then the internal polling sequence determines which request is to be serviced.

IP Register

- IP.6 Reserved for future use.
- IP.5 Reserved for future use.
PS IP.4 It defines the serial port interrupt priority level.
PT1 IP.3 It defines the timer interrupt of 1 priority.
PX1 IP.2 It defines the external interrupt priority level.
PT0 IP.1 It defines the timer0 interrupt priority level.
PX0 IP.0 It defines the external interrupt of 0 priority level.

TCON Register

TCON register specifies the type of external interrupt to the microcontroller.