I have set a pin on a Cortex M3 to act as an rising-edge triggered interrupt and it's not firing for some reason. What it should do is receive a 1Hz pulse, trigger an interrupt and start a 12.8Khz square wave.

__main and EINT3_IRQHandler are defined in startup code created by the program, Keil uVision 5. The GPIO interrupts share the EINT3 interrupt vector with a different external interrupt. I figured it would only be a matter of activating the GPIO and EINT3 interrupts then putting the handler within the EINT3_IRQHandler label.

The GPIO interrupts are described on page 129 of this data sheet.


I also tried to just set it to read the interrupt status by using

    CMP [status_register], #1   ; Check interrupt status
    BEQ wave_gen                ; If triggered, start square wave
    BNE check                   ; else keep checking

and use that as a trigger but that didn't work either.

This is my code.

    EXPORT __main   
    LDR     R1,=0x2009c000      ; Pointer to base of port 0 
    LDR     R2,=0x00000001      ; To control pin 0 for square wave
    LDR     R3,=0x00000001      ; For XOR to invert pin 0
    STR     R2,[R1,#0x00]       ; Set pin 0 to output mode (base+0x00)
    STR     R2,[R1,#0x14]       ; Pin 0 HIGH
    LDR     R4,=780             ; Set up timer

    LDR     R5,=0x40028080      ; GPIO Rising Edge Interrupt
    LDR     R7,=0xE000E100      ; Set EINT3-enable register

    LDR     R6,=0x00000002      ; For pin 0:1
    STR     R6,[R5,#10]         ; Activate 0:1 as rising edge interrupt

    LDR     R6,=0x00200000      ; To enable ENINT3
    STR     R6,[R7]             ; Enables EINT3
    SUB     R4, #1              ; Decrement timer
    CMP     R4, #0              ; Compare to 0
    BNE     wave_gen            ; If !0, keep looping
    LDR     R4,=975             ; Reset timer
    EOR     R2,R3               ; Invert bits
    STR     R2,[R1,#0x14]       ; Invert pin
    MUL     R3,R3,R3            ; To waste cycles, ignore (1*1=1)
    MUL     R3,R3,R3
    B       wave_gen            ; Restart loop
    B       wave_gen    ; Start square wave
  • \$\begingroup\$ If you poll the status of the input pin (i.e. read the FIO0PIN register), does the value of the input pin ever change? Perhaps you need to configure PINMODE0 or PINMODE_OD0 registers. \$\endgroup\$
    – kkrambo
    Commented Apr 29, 2014 at 20:42
  • \$\begingroup\$ You'll drive yourself bats coding a cortex M3 in assembly \$\endgroup\$ Commented Dec 16, 2014 at 11:31

1 Answer 1


I haven't really done assembly with the cortex M3, but as far as I can tell you're not enabling the peripheral clock for whichever GPIO peripheral you are using - thats what got me the first time I did it, and what seems to catch lots of people out.


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