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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.

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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:

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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

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Circuit Diagram:

Source Code: