An interrupt is an asynchronous signal that needs attention. An interrupt stops the CPU of a microcontroller, leaving the tasks that it is currently doing, to give attention to the interrupt signal. Once the attention has been given to the interrupt signal, the CPU goes back to its unaccomplished task before the interrupt has occured and continues the task.

The Interrupts of AT89C2051

The interrupts of AT89C2051 is compatible with the interrupts of the original 8051 microcontroller. It has 6 interrupts sources ( 5 interrupts + RESET). The interrupt sources of AT89C2051 are the following:

Please note that in this tutorial, we will not consider the interrupt from RESET.

The table above shows the interrupt sources of AT89C2051 and their respective interrupt priority number. Knowing the priority number is important specially if two different interrupt sources occur at the same time.

Interrupt Enable Register

The Interrupt Enable (IE) register is responsible in enabling and disabling the different interrupt sources of 8051.

How To Enable an Interrupt

1. Initialize the sources of interrupts such as Timers, External Interrupts, or UART.

2. Set the bits of the IE register that corresponds to the interrupt sources that you want to be enabled.

Example: If you want to enable the interrupt of the serial port or UART set ES to 1 or ES=1.

3. Enable the global interrupt by setting the EA bit of the IE register (EA=1).

How to Write an interrupt service routine or ISR

The interrupt service routine or ISR is the routine that an MCU is servicing every time an interrupt occurs. It can be treated as an ordinary subroutine in a C program.

The format of the ISR is:

void your_ISR_name(void) interrupt interrupt_priority_number
{
  //your routine here
}

The your_ISR_name is user defined. It can be any name.

The interrupt_priority_number is fixed depending on the source of the interrupt. Refer to the table of the interrupt sources above for reference.