Interrupt Subsystem

A Dr. Taylor Tutorial

Interrupt Subsystem

      sbic  ADCSRA, ADSC  ; skip if bit clear (ADC complete)
      rjmp  poll
      in    r16, ADCH

Interrupt Sources

The Interrupt Subsystem on the ATmega32 can receive interrupts from internal and external sources.

Internal Interrupt Sources

External Interrupt Sources

Interrupt Vector Table

.org 0x00
     rjmp  start

; Interrupt Vector Table

.org 0x2a
; ....
.org 0x00
     rjmp  start
.org ADCCaddr
     rjmp  adcISR
.org 0x2a
     ; ...

; ISR for ADC subsystem
     ; ...

The ATmega32 interrupt subsystem supports twenty one different kinds of interrupts:

Vector No. Program Address Label Source Interrupt
1 0x00 RESET Power-on Reset, etc...
2 0x02 INT0addr INT0 External Interrupt Request 0
3 0x04 INT1addr INT1 External Interrupt Request 1
4 0x06 INT2addr INT2 External Interrupt Request 2
5 0x08 OC2addr TIMER2 COMP Timer/Counter2 Compare Match
6 0x0A OVF2addr TIMER2 OVF Timer/Counter2 Overflow
7 0x0C ICP1addr TIMER1 CAPT Timer/Counter1 Capture Event
8 0x0E OC1Aaddr TIMER1 COMP A Timer/Counter1 Compare Match A
9 0x10 OC1Baddr TIMER1 COMP B Timer/Counter1 Compare Match B
10 0x12 OVF1addr TIMER1 OVF Timer/Counter1 Overflow
11 0x14 OC0addr TIMER0 COMP Timer/Counter0 Compare Match
12 0x16 OVF0addr TIMER0 OVF Timer/Counter0 Overflow
13 0x18 SPIaddr SPI STC Serial Transfer Complete
14 0x1A URXCaddr USART RXC USART, Rx Complete
15 0x1C UDREaddr USART UDRE USART, Data Register Empty
16 0x1E UTXCaddr USART TXC USART, Tx Complete
17 0x20 ADCCaddr ADC ADC Conversion Complete
18 0x22 ERDYaddr EE_RDY EEPROM Ready
19 0x24 ACIaddr ANA_COMP Analog Comparator
20 0x26 TWIaddr TWI Two-wire Serial Interface
21 0x28 SPMRaddr SPM_RDY Store Program Memory Ready

Enabling Interrupts

Interrupt Processing Steps


When the interrupt occurs:

Interrupt Service Routine Guidelines

    push   r0
    in     r0, SREG
    push   r0
; ...
    pop    r0
    out    SREG, r0
    pop    r0

Sample Code

Here is a simple program that makes use of the interrupt subsystem to display the results of the ADC conversion on the LEDs.

; Author: t a y l o
; Date: 4-23-2007
; Filename: adcIntDemo.asm
; This program uses the ADC subsystem and the interrupt subsystem
;  to take the analog signal on ADC channel 0 and display its digital
;  representation on the LEDs.
; Hardware configuration:
;  - PORTB connected to LEDs.
;  - PORTA pin 0 connected to analog source.

.include ""

.def temp = r16  ; Use r16 as a temp register

.org 0x00
     rjmp  stackInit
.org ADCCaddr     ; Jump vector for ADC interrupts
     rjmp  adcISR

.org 0x2a
     ldi   temp, HIGH(RAMEND-0x20)
     out   SPH, temp
     ldi   temp, LOW(RAMEND-0x20)
     out   SPL, temp

     ldi   temp, 0xff   ; Configure PORTB as output
     out   DDRB, temp
     cbi   DDRA, 0      ; Configure PORTA as input
     rcall adcIntInit0
     sbi   ADCSRA, ADSC ; Start AD conversion

; Program just loops forever doing nothing (until interrupted)
     rjmp  forever

; Initialize ADC for single ended conversion on ADC channel 0
;  with interrupt subsystem enabled.
.def temp = r16
     push  temp

     ; Set ADC reference voltage, put in 8-bit mode and select channel 0
     ldi   temp, 0b01100000
     out   ADMUX, temp

     ; Enable ADC and ADC interrupts, prescale factor = 128
     ldi   temp, 0b10001111
     out   ADCSRA, temp
     sei                ; Enable global interrupt flag

     pop   temp

; Interrupt Service Routine for ADC subsystem
; Takes result of ADCH and sends compliment of the result
;  (since LED are active low) to the LEDs on PORTB and then
;  starts the ADC all over again.
; Assumptions:
;  - ADC and Interrupt subsystems have been initialized.
;  - Using ADC channel 0.
;  - Using only 8 MSB of ADC result.
;  - PORTB is configured as output.
; Hardware configuration:
;  - Analog input source is connected to PORTA pin 0.
;  - PORTB connected to active low LEDs.
     push  temp
     in    temp, SREG   ; Capture status register
     push  temp

     in    temp, ADCH
     out   PORTB, temp
     sbi   ADCSRA, ADSC ; Start ADC again

     pop   temp
     out   SREG, temp   ; Restore status register
     pop   temp

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