External Interrupt Using AVR Microcontroller (Atmega-8)

Aim:

Although micro controllers can accept inputs from Digital I/O Ports,interrupts are preferred for accepting inputs generated by external events. This is mainly because of the less attention required for an interrupt based program. An Interrupt Event directs the flow of program execution to a totally independent piece of code, known as “Interrupt Sub-Routine”. There are many sources of Interrupts that are available for a micro controller. Most of them are generated by the internal modules and are known as internal interrupts. And there are two pins that accept external signals directly and allows external sources to generate interrupts.

Description:

External interrupts are fairly powerful, they can be configured to trigger on one of 4 states.  Low level will trigger whenever the pin senses a LOW (GND) signal.  Any Logic Change trigger at the moment the pin changes from HIGH (Vcc) to LOW (GND) or from LOW (GND) to HIGH(Vcc). On Falling Edge will trigger at the moment the pin goes from HIGH (Vcc) to LOW (GND). On Rising Edge will trigger at the moment the pin goes from LOW (GND) to HIGH (Vcc).  The best part is that you can configure each INTx independently.

External interrupts use the below 3 registers.  Which you could find under the “External Interrupts section of the datasheet.

	7 bit	6 bit	5 bit	4 bit	3 bit	2 bit	1 bit	0 bit
MCUCR	SE	SM2	SM1	SM0	ISC11	ISC10	ISC01	ISC00

MCU Control Register

ISCx1	ISCx0	DESCRIPTION
0	0	Low level of INTx generates an interrupt request
0	1	Any logic change on INTx generates an interrupt request
1	0	The falling edge of INTx generates an interrupt request
1	1	The rising edge of INTx generates an interrupt request

ISC Bits Settings

	7 bit	6 bit	5 bit	4 bit	3 bit	2 bit	1 bit	0 bit
GICR	INT1	INT0	–	–	–	–	IVSEL	IVCE

General Interrupt Control Register

	7 bit	6 bit	5 bit	4 bit	3 bit	2 bit	1 bit	0 bit
GIFR	INTF1	INTF0	–	–	–	–	–	–

General Interrupt Flag Register

When the event specified by the Interrupt Sense Control (ISCxy) bits in the MCU Control Register (MCUCR) is sensed on the INTx pin, the External Interrupt Flag x (INTFx) in the General Interrupt Flag Register (GIFR) is set HIGH (1).  If the I-bit in SREG and the INT1 bit in the GICR are both HIGH(1) the MCU will jump to the corresponding vector.  The Flag is set to LOW (0) when the INTx_vect routine is executed.

Block Diagram

Schematic

Code

// ******************************************************
// Project: Interfacing external interrupt to atmega8
// Author: Hack Projects India
// Module description: Operate single LED
// ******************************************************

#define F_CPU 8000000UL
#include <avr/io.h>
#include<util/delay.h>

#include <avr/interrupt.h>

ISR(INT0_vect)
{
 cli();
 PORTB=0x01;
 _delay_ms(1000);
 PORTB=0x00;
 _delay_ms(1000);
 sei();
}
int main(void)
{
 DDRB=0xff;
 DDRD=0x00;
 MCUCR=0x03;
 GICR=(1<<INT0);
 SREG=0x80;
 while(1);
}

Downloads:

The code was compiled in Atmel Studio 6 and simulation was made in Proteus v7.7.
To download code and proteus simulation click here.

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