ECE 2325 UMD S.Norr

HW6: 11-7: An alternate method toggling PA5 using the

CFORC register:

;

; Dly10: a subroutine which turns on PA5,

; waits for 10 mS, then turns off PA5

; and returns.

;

; DOES NOT CORRUPT REGISTERS

;

; Assumptions:

; PR1, PR0 = 1,0 and E-clck = 2 MHz such that

; TCNT is counting at 4 microsec per count

; Also assume that PA5 is currently off

Dly10: pshx

psha

pshb

ldx #1000

bset 26,x,80 ; Make PortA = output (DDRA = 1)

ldaa 20,x

psha ; Put old contents of TCTL1 on stack

bset 20,x,10 ;

bclr 20,x,20 ; Make OM3, OL3 = 01 (Toggle PA5)

bset 0b,x,20 ; Force an OC3 using CFORC register

ldd 0e,x

addd #9c0 ; Set up TOC3 for 10 ms compare :

std 1a,x ; (9c4 - 4) cycles which executed

; between PA5 on and the load of TCNT

bclr 23,x,20 ; clear any "stale" OC3F flag to

; prepare for new one

Wait: brset 23,x,20,End ; Wait for OC3F flag to be set

bra Wait

End: pula

staa 20,x ; Restore old contents of TCTL1

pulb

pula ; restore accumulators and return

pulx

rts