delay.s

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;
; Delay subroutines
;
; by Fernando L. Garcia Bermudez
;
; v.1.0
;
; Revisions:
;  Fernando L. Garcia Bermudez     2007-9-14    Mixed assembly-C implementation
;                                  2010-7-23    Full assembly implementation
;

#include p33Fxxxx.inc

; Symbol definitions
.equiv  time, w0
.equiv  temp, w2


;------------------------------------------------------------------------------
;           Public functions
;------------------------------------------------------------------------------

.global _delay_us
.global _delay_ms

; Need to waste ~40 inst/us (for 40MIPS) and since assembly loop takes 5
; instructions, it should be repeated 8 times to elapse a us. Since temp is a
; 16-bit word, the maximum delay possible is ~8.2ms (time=8192).
_delay_us:

            mul.uu  time, #8, temp
            sub     #2, temp        ; #2 obtained testing btg delays
ELAPSED:    nop
            nop
            dec     temp, temp
            bra     NZ, ELAPSED

            return

; The inner loop, modelled after the _delay_us routine, is repeated "time"
; times wasting 1ms per iteration (1000us * 8 - 2 = 7998).
_delay_ms:

msELAPSED:  mov     #7998, temp
usELAPSED:  nop
            nop
            dec     temp, temp
            bra     NZ, usELAPSED
            dec     time, time
            bra     NZ, msELAPSED

            return


.end