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3. External Interrupt Registers

For detailed information about the registers, please refer to the Atmega32p datasheet. here

From the previous section, we have set up the interrupt handler for external interrupt 0. Now, we will set up the registers to setup/initialize this interrupt. Let's say we want to toggle an LED on pin PB5 with falling edge trigger. We can write the following code in our setup section:

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

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

setup:
    ; initialize external interrupt 0
    ldi r16, (1<<ISC01) ; Set ISC01 bit = 1 | trigger on falling edge
    out EICRA, r16 ; write to EICRA register to set the interrupt trigger condition
    ldi r16, (1<<INT0) ; set INT0 bit = 1 | enable external interrupt 0
    out EIMSK, r16 ; write to EIMSK register to enable the interrupt

    ; enable global interrupts
    sei

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

Let's break it down register by register:

EICRA (External Interrupt Control Register A)

  • This register is used to configure the trigger condition for external interrupts. For INT0, we need to set the ISC01 bit to 1 and ISC00 bit to 0 for falling edge trigger. This is done by loading the value (1<<ISC01) into register r16 and then writing it to EICRA.

EIMSK (External Interrupt Mask Register)

  • This register is used to enable or disable external interrupts. To enable INT0, we need to set the INT0 bit to 1. This is done by loading the value (1<<INT0) into register r16 and then writing it to EIMSK.

sei (Set Global Interrupt Enable)

This instruction enables global interrupts. It MUST be called after setting up the individual interrupt configurations to allow the microcontroller to respond to interrupts.

You should get the rough idea for INT1 as well. You just need to set the ISC11 and ISC10 bits in EICRA for the trigger condition and set the INT1 bit in EIMSK to enable it. Read the datasheet for more details on the trigger conditions for INT1.

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