Diff: sensor_setup.s

Revision:

1:f9f0b92a9d7c

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/sensor_setup.s	Mon Feb 23 23:10:36 2015 +0000
@@ -0,0 +1,61 @@
+  AREA asm_func, CODE, READONLY
+; Export my asm function locaiton so that the C compiler can find it and link
+    EXPORT start_systick
+    EXPORT measure_clock_cycles
+;   
+    ALIGN
+;   
+start_systick   
+;   
+; Setup base address for SysTick
+    LDR r2, =0xE000E010 
+; Setup SysTick and start
+    LDR r0, [r2, #0x00]     ; Grab current SysTick control state
+    LDR r1, =0x05           ; Mask to set SysTick clock source as processor clock and start it running
+    ORRS r0, r0, r1
+    STR r0, [r2, #0x00]
+; Return to previous code
+    BX  LR
+;
+measure_clock_cycles
+;
+    PUSH {r4-r7}
+; Configure pin to sample from
+    LDR r3, =0xF80FF094         ; Port C data direction register
+    LDR r4, =0x00000020         ; Pin 5
+    LDR r5, [r3]                ; Current config
+    BICS r5, r5, r4             ; Turn PinC5 to 0, General Purpose Input
+    STR r5, [r3]                ; Save config 
+; Setup base address for current SysTick value
+    LDR r5, =0xE000E018
+; Sample starting SysTick   
+    LSLS r0, r0, #2             ; Multiply by 2 as both rising and failing edges detected
+    LDR r0, =0x00               ; Clear counter
+    LDR r2, =0x20               ; Pin mask
+    LDR r6, =0xF80FF090         ; Input register
+    LDR r7, =0x00               ; Current pin state
+    LDR r4, [r5]                ; Read current SysTick
+loop
+    LDR r3, [r6]                ; Read Input register
+    ANDS r3, r3, r2             ; Mask out pin
+    SUBS r3, r3, r7             ; Sneaky method of detecting bit change, equals zero when no change has occurred
+    BEQ loop                    ; Loop Until change
+    MOV r7, r3                  ; Store new pin state  
+    ADDS r0, #1                 ; Increment counter by one
+    SUBS r3, r3, r0             ; Subtract SAMPLES from counter
+    BNE loop                    ; Loop until counter = samples
+    LDR r1, [r5]                ; Read ending SysTick
+; Compute time difference
+    MOV r0, r4
+    SBCS r0, r0, r1             ; r0 = r4 - r1
+    BPL finish                  ; If result of subtraction creates a positive number, return r0, otherwise recalculate
+    LDR r3, =0x00FFFFFF
+    SUBS r0, r3, r1             ; r0 = (2^24 - 1 - r1) + r4
+    ADDS r0, r0, r4
+finish
+; Return to previous code
+    POP {r4-r7}
+    BX LR
+    ALIGN
+    END
+           
\ No newline at end of file