2. Interrupt Handler

On the ATMega328P microcontroller, there are three essential requirements that must met to enable an interrupt:

1. Global Interrupt Enabled: Interrupts must be allowed to occur globally across any part of the program. While the Arduino environment typically enables this by default, it is important to verify this when using different microcontrollers or during troubleshooting.
2. Individual Interrupt Enabled: Each specific interrupt must be permitted to occur via dedicated interrupt control registers.
3. Interrupt Condition Met: The microcontroller must receive a signal (either internal or external) that matches the predefined trigger criteria.

To enable a microcontroller to process interrupts, several steps must be followed. This module organizes these steps according to their placement in the code for easier implementation. The first step involves preparing the Interrupt Handler the specific code that will run when the interrupt is triggered.

Microcontrollers use a specialized memory block called the Interrupt Vector Table. This table stores the memory addresses of the programs to be executed for each specific type of interrupt.

These addresses are fixed and cannot be changed. When an interrupt occurs, the microcontroller automatically jumps to the corresponding address in the Interrupt Vector Table to execute the associated program.

For example if an external interrupt (INT0) is triggered, the microcontroller will jump to address 0x0002 to execute the code defined in that location.

See the program address between INT0 and INT1, which are 0x0002 and 0x0004 respectively. The gap between these addresses is only 2 bytes, which is the size of a single instruction in AVR assembly language. This means that the interrupt handler for INT0 must be concise and fit within this limited space, often requiring the use of a jump instruction to redirect to a larger block of code if necessary.

Example:

#define __SFR_OFFSET 0x00
#include "avr/io.h"

.cseg ; program memory segment
.org 0x0002	 ; external interrupt 0 vector
    rjmp toggle_led

toggle_led:
    ; main interrupt logic here
    in r16, PORTB
    eor r16, (1<<PB5)
    out PORTB, r16
    reti ; return from interrupt

From the above example, we have initialized the handler for the interrupt, and then created a routine named toggle_led, which contains the code that will be executed when the interrupt is triggered. When an INT0 occurs (for example, when a button connected to the INT0 pin is pressed), the microcontroller will jump to the toggle_led routine. After performing the necessary actions, we need to inform the microcontroller that the interrupt has been successfully handled and to continue with the main program. We can use the keyword RETI, which stands for Return from Interrupt, to achieve this.

An interrupt will take priority over the main program, meaning that when an interrupt occurs, the microcontroller will temporarily halt the execution of the main program to execute the interrupt handler. Programmer must be cautious when writing interrupt handlers, as they should be efficient and not contain long-running code, to avoid delaying the main program for too long.