LED Control ARM

start
            BL longpulse
            BL longpulse
            BL longpulse
            BL shortpulse
            BL shortpulse
            BL shortpulse
            B start

longpulse   PUSH {LR}
            BL ledon
            MOV R0,#2000
            BL delay

            BL ledoff
            MOV R0,#1000
            BL delay

            POP {LR}
            BX LR

shortpulse  PUSH {LR}
            BL ledon
            MOV R0,#200
            BL delay

            BL ledoff
            MOV R0,#1000
            BL delay

            POP {LR}
            BX LR

; delay subroutine - blocks for the number of milliseconds in R0.
delay   PUSH {lr}
        PUSH {R0}
        PUSH {R1}
delayo  MOV R1,#3200        ; Start of outer loop, counts down from R0.
delayi  SUB R1,#0x01        ; Start of inner loop, counts down from 3200 (takes ~1ms).
        CMP R1,#0x00
        BNE delayi          ; End of inner loop
        SUB R0,#1           ; Subtract 1 from number of milliseconds.
        CMP R0,#0           ; Check if we got to 0ms.
        BNE delayo          ; If not, go around the inner loop again...
        POP {R1}
        POP {R0}
        POP {lr}
        BX  LR                  ; Otherwise return.

; ledon subroutine - switches the LED on.
ledon
        PUSH {lr}
        PUSH {R1}
        PUSH {R2}
        PUSH {R0}
        LDR R0,=0x40020000  ; Put GPIOA register Base Address in R0 (see page 38 RM0368).
        LDR R1,[R0, #0x14]  ; Load the current value of port A ODR (base+0x14) into R1.
        MOV R2,#1               ; 1 of 3 lines to do a 'R1=R1|(1<<5)'
        LSL R2,#5               ; 2 of 3 lines to do a 'R1=R1|(1<<5)'
        ORR R1,R2                   ; 3 of 3 lines to do a 'R1=R1|(1<<5)'
        STR R1,[R0, #0x14]  ; Store new value for port A ODR with 5th bit set.
        POP {R0}
        POP {R2}
        POP {R1}
        POP {lr}
        BX  LR

; ledoff subroutine - switches the LED off.
ledoff  PUSH {lr}
        PUSH {R1}
        PUSH {R2}
        PUSH {R0}
        LDR R0,=0x40020000  ; Put GPIOA register Base Address in R0 (see page 38 RM0368).
        LDR R1,[R0, #0x14]  ; Load the current value of port A ODR (base+0x14) into R1.
        MOV R2,#1               ; 1 of 3 lines to clear the 5th bit of R1.
        LSL R2,#5               ; 2 of 3 lines to clear the 5th bit of R1.
        BIC R1,R2                   ; 3 of 3 lines to clear the 5th bit of R1.
        STR R1,[R0, #0x14]  ; Store new value for port A ODR with 5th bit set.
        POP {R0}
        POP {R2}
        POP {R1}
        POP {lr}
        BX  LR