Interrupt Subsystem

Interrupt Subsystem

• We have two strategies for getting input from an external device:
  • Polling — continually watch for completion of an event.
  • Interrupts — hardware generated subroutine call.

• Polling with the ADC involved a tight loop that watched for a change in the ADSC flag:

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

• Using interrupts allows the microcontroller to continue executing useful instructions.
• When an interrupt occurs:
  • Completes current instruction.
  • Saves program counter (PC) on the stack.
  • Executes interrupt jump vector instruction (typically loads PC with the location of the Interrupt Service Routinue starting location).
  • Executes Interrupt Service Routine (ISR).
  • Returns to program when ISR encounters RETI instruction.

Interrupt Sources

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

Internal Interrupt Sources

• Power Subsystem:
  • Power on
  • Reset
  • Voltage low
• Timer/Counter Subsystem:
  • Timer expired
  • Counter match
• Analog to Digital Converter Subsystem:
  • ADC complete
• Serial Communication Subsystem:
  • Character received
  • Character sent
  • Covered in more detail in CS-3841: Design of Operating Systems.

External Interrupt Sources

• Anything that can generate an "on/off" signal.
• ATmega32 has 3 pins (PD2, PD3, PB2) that the controller "listens" for an external trigger.

Interrupt Vector Table

.cseg
.org 0x00
     rjmp  start

; Interrupt Vector Table

.org 0x2a
start:
; ....

• The ATmega32 has approximately 20 interrupts.
• Each interrupt has a program memory location that stores a RJMP instruction that jumps to the ISR for the particular interrupt.
• The m32def.inc file has labels associated with each interrupt type
• Setting up the location of the ISR for the ADC, looks like this:

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

; ISR for ADC subsystem
adcISR:
     ; ...

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

Enabling Interrupts

• Must enable the global interrupt flag.
  • SREG has an I flag.

  

• Ways to set I flag (enable interrupts):
  • SEI — Set Enable Interrupt.
  • RETI — RETurn from Interrupt service routine (automatically sets the I flag.
• Ways to clear I flag (disable interrupts):
  • CLI — clears the interrupt flag.
  • Whenever the ATmega32 is reset, the I flag is cleared.
  • Whenever an interrupt occurs, the I flag is cleared.

Interrupt Processing Steps

Setup:

• Must initialize the stack.
• Must set up the interrupt jump table.
• Must enable the global interrupt flag.
• Must enable particular local interrupt we are interested in.

When the interrupt occurs:

• Finish current instruction.
• Push current PC onto the stack.
• Clear I flag.
• Execute RJMP instruction in the Interrupt Vector Table.
• Execute the interrupt service routine.
• Execute RETI at end of ISR.
• Set I flag.
• Pop saved PC into the PC.

Interrupt Service Routine Guidelines

• Make it as short as possible.
• Push/Pop any registers that must be preserved onto the stack.
• Push/Pop SREG onto the stack.

    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 r@msoe.edu
; 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.

.nolist
.include "m32def.inc"
.list

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

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

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

start:
     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)
forever:
     rjmp  forever

; Initialize ADC for single ended conversion on ADC channel 0
;  with interrupt subsystem enabled.
adcIntInit0:
.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
     ret

; 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.
adcISR:
     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
     reti