Interrupts in ARMlite

Interrupts in ARMlite

Revisiting our Flashing LED Example

• Recall our flashing LED program from last week, and in particular the delay function:

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; delay function

;; inputs R0 - time delay in seconds

delay:

push {R3,R4,R5,R6}
MOV R3, R0 ; move delay time param into R3
LDR R4, .Time ; get start ime

timer:

LDR R5, .Time ; update time
SUB R6, R5, R4 ; calc elapsed time
CMP R6, R3 ; compare elapsed to delay time

BLT timer

pop {R3,R4,R5,R6}

RET

Revisiting our Flashing LED Example

• This is a busy-wait timer, or in other words, an example of polling:

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; delay function

;; inputs R0 - time delay in seconds

delay:

push {R3,R4,R5,R6}
MOV R3, R0 ; move delay time param into R3
LDR R4, .Time ; get start ime

timer:

LDR R5, .Time ; update time
SUB R6, R5, R4 ; calc elapsed time
CMP R6, R3 ; compare elapsed to delay time

BLT timer

pop {R3,R4,R5,R6}

RET

Revisiting our Flashing LED Example

• The CPU constants checks for a state – in this case, that the time exceeds a delay time.;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; delay function;; inputs R0 – time delay in seconds delay: push {R3,R4,R5,R6} MOV R3, R0; move delay time param into R3 LDR R4,.Time; get start ime timer: LDR R5,.Time; update time SUB R6, R5, R4; calc elapsed time CMP R6, R3; compare elapsed to delay time BLT timer pop {R3,R4,R5,R6} RET

Revisiting our Flashing LED Example

• The CPU is 100% engaged in this task! It is doing nothing else but wasting CPU cycles;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; delay function;; inputs R0 – time delay in seconds delay: push {R3,R4,R5,R6} MOV R3, R0; move delay time param into R3 LDR R4,.Time; get start ime timer: LDR R5,.Time; update time SUB R6, R5, R4; calc elapsed time CMP R6, R3; compare elapsed to delay time BLT timer pop {R3,R4,R5,R6} RET

Revisiting our Flashing LED Example

• There is a better way! Using Interrupts

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; delay function

;; inputs R0 - time delay in seconds

delay:

push {R3,R4,R5,R6}
MOV R3, R0 ; move delay time param into R3
LDR R4, .Time ; get start ime

timer:

LDR R5, .Time ; update time
SUB R6, R5, R4 ; calc elapsed time
CMP R6, R3 ; compare elapsed to delay time

BLT timer

pop {R3,R4,R5,R6}

RET

Now back to our Flashing LED

• Lets use ARMlite’s Clock Interrupt for this!
• To set itup the interrupt handling:
• The handler will be called everytime the clock raises an interrupt
• So each time it will draw either a green (“LED on”) pixelor a white (“LED off”) depending on what state the LED is currently in
• This is quite different to previously where we inserted a delay between each state change!
• Now we’re waiting to be told when to change state:
• Event driven!

Flashing LED with INT: Interrupt Handler

///////////////////////////////// /// interrupt handler: toggleLED // toggles state of LED (on "green" or off "white") // toggleLED:

PUSH {R0}

CMP R3, #0 // check state

BNE off

MOV R3, #1 // if R3 0, then 1
MOV R0, #.green

B drawpixel off:

MOV R3, #0 // if R3 1, then 0
MOV R0, #.white

drawpixel:

STR R0, .Pixel367 // draw the pixel
POP {R0}

RFE

Flashing LED with INT: Interrupt Handler

///////////////////////////////// /// interrupt handler: toggleLED // toggles state of LED (on "green" or off "white") // toggleLED:

CMP R3, #0 // check state Check next state to enact (on or off)

BNE off

MOV R3, #1 // if R3 0, then 1
MOV R0, #.green

B drawpixel off:

MOV R3, #0 // if R3 1, then 0
MOV R0, #.white

drawpixel:

STR R0, .Pixel367 // draw the pixel
POP {R0}

RFE

Flashing LED with INT: Interrupt Handler

///////////////////////////////// /// interrupt handler: toggleLED // toggles state of LED (on "green" or off "white") // toggleLED:

CMP R3, #0 // check state

BNE off

MOV R3, #1 // if R3 0, then 1
MOV R0, #.green

B drawpixel off:

MOV R3, #0 // if R3 1, then 0 next state is “off” ?
MOV R0, #.white Then turn LED off

drawpixel:

STR R0, .Pixel367 // draw the pixel
POP {R0}

RFE

Flashing LED with INT: Interrupt Handler

///////////////////////////////// /// interrupt handler: toggleLED // toggles state of LED (on "green" or off "white") // toggleLED:

CMP R3, #0 // check state

BNE off

MOV R3, #1 // if R3 0, then 1 next state is “ON” ?
MOV R0, #.green Then turn LED ON

B drawpixel off:

MOV R3, #0 // if R3 1, then 0
MOV R0, #.white

drawpixel:

STR R0, .Pixel367 // draw the pixel
POP {R0}

RFE

Flashing LED with INT: Interrupt Handler

///////////////////////////////// /// interrupt handler: toggleLED // toggles state of LED (on "green" or off "white") // toggleLED:

CMP R3, #0 // check state

BNE off

MOV R3, #1 // if R3 0, then 1
MOV R0, #.green

B drawpixel off:

MOV R3, #0 // if R3 1, then 0
MOV R0, #.white

drawpixel:

STR R0, .Pixel367 // draw the pixel Draw the pixel (R0 will be set as
POP {R0} either green or white)

RFE

Flashing LED with INT: Interrupt Handler

///////////////////////////////// /// interrupt handler: toggleLED // toggles state of LED (on "green" or off "white") // toggleLED:

CMP R3, #0 // check state We push and popR0 so we can

BNE off restoreitafterthefunctionis

MOV R3, #1 // if R3 0, then 1 finished.
MOV R0, #.green

B drawpixel We don’tdo thisforR3though – off: why?

MOV R3, #0 // if R3 1, then 0
MOV R0, #.white

drawpixel:

STR R0, .Pixel367 // draw the pixel
POP {R0}

RFE

Flashing LED with INT: Full Program

Program 11|/////////////////////////////////

1|// Set up Interrupt handling 12|/// interrupt handler: toggleLED

2| MOV R0,#toggleLED 13|// toggles state of LED (on "green" or off "white")
3| STR R0,.ClockISR 14|//
4| MOV R0,#1000 15|toggleLED:
5| STR R0,.ClockInterruptFrequency 16| PUSH {R0}
6| MOV R0,#1 17| CMP R3, #0 // check state
7| STR R0,.InterruptRegister //Enable all interrupts 18| BNE off
8| MOV R3, #1 // R3 keeps track of state of LED 19| MOV R3, #1 // if R3 0, then 1
20| MOV R0, #.green

9|mainProgram: 21| B drawpixel 10| B mainProgram //Here, just an empty loop! 22|off:

23| MOV R3, #0 // if R3 1, then 0
24| MOV R0, #.white

25|drawpixel:

26| STR R0, .Pixel367 // draw the pixel
27| POP {R0}

28| RFE

Flashing LED with INT: Full Program

1|// Set up Interrupt handling 12|/// interrupt handler: toggleLED

2| MOV R0,#toggleLED 13|// toggles state of LED (on "green" or off "white")
3| STR R0,.ClockISR 14|//
4| MOV R0,#1000 15|toggleLED:
5| STR R0,.ClockInterruptFrequency 16| PUSH {R0}
6| MOV R0,#1 17| CMP R3, #0 // check state
7| STR R0,.InterruptRegister //Enable all interrupts 18| BNE off
8| MOV R3, #1 // R3 keeps track of state of LED 19| MOV R3, #1 // if R3 0, then 1
20| MOV R0, #.green

9|mainProgram: 21| B drawpixel 10| B mainProgram //Here, just an empty loop! 22|off:

23| MOV R3, #0 // if R3 1, then 0
This is just an infinite loop because we are only 24| MOV R0, #.white

flashing LED and there is nothing else to do but wait 25|drawpixel:

for clock interrupts, however this could potentially be 26| STR R0, .Pixel367 // draw the pixel
doing other useful things 27| POP {R0}

28| RFE

Flashing LED with INT: Full Program

• Lets look in ARMlite