External Interrupts in AVR Microcontroller

Microcontrollers can accept inputs from I/O ports, interrupts are used for accepting inputs generated by external events. Interrupt event directs the flow of program execution with a totally independent piece of code, known as "Interrupt Sub-Routine". There are many sources of interrupts that are available for a microcontroller. Most of them are generated by internal modules and are called as internal interrupts.

For running an interrupt subroutine following requirements are necessary:

The interrupt source must be activated by setting the corresponding interrupt mask/ Interrupt Enable Bit.
The enable bit in AVR status register must be set to 1. For this the instruction named 'sei' (Set Interrupt Enable).
The interrupt sub routine must present. If there is no code to b e run, then an empty subroutine must occur at particular memory spaced to that interrupt.
Finally the event must occur, so the execution of the routine gets triggered.

Writing an Interrupt Subroutine in AVR Studio:

It is tricky to use an interrupt subroutine into a C code of a microcontroller. Therefore the AVR GCC developers use a few symbols to represent the interrupts and macros that minimized the code size in many programs.

The interrupt subroutine for External Interrupt 0 and External Interrupt 1 is given below:

// Interrupt sub routine code starts
ISR(INT1_vect)
{
// Code for interrupt 1
}
ISR(INT0_vect)
{
// Code for interrupt 0
}
// Interrupt Subroutine Code Ends

Registers Associated with External Interrupts:

The table showing microcontroller unit control register is given below:

MCU Control Register- MCUCR
Bit	7	6	5	4	3	2	1	0
Bit Name	SE	SM2	SM1	SM0	ISC11	ISC10	ISC01	ISC00
Initial value	0	0	0	0	0	0	0	0
Read/Write	RW	RW	RW	RW	RW	RW	RW	RW

The table showing Interrupt Sense Control truth table is:

ISCx1	ISCx0	Interrupt Generated Upon
0	0	The lower Level of INTx pin
0	1	Any logical change inside INTx pin
1	0	Falling edge of INTx pin
1	1	Rising edge of INTx pin

Circuit Diagram:

Source Code:

#include <avr/io.h>
ISR(INT0_vect)
{
PORTB &=0xFE;
}
ISR(INT1_vect)
{
PORTB |=0x01;
}
void initInterrupt()
{
GICR =(1<<INT0)|(1<<INT1);
MCUCR() =(1<<ISC11)|(1<<ISC01);
sei();
}
int main()
{
DDRB=0xFF;
initInterrupt();
while(1);
return 1;
}

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