8.4. 500 HZ TONE GENERATOR 89
SEI
LDD #1000
STD ONTIME
LDD #3000
STD OFFTIME
LDAA #%01000000
STAA TMSK1
CLI
; Now go about your business of printing Z’s
LDAA #’Z’
LOOP JSR OUTA
BRA LOOP
; End of main program
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; INTERRUPT SERVICE
SERVICE
; TOGGLE PA4
LDAA #%00010000
EORA PORTA
STAA PORTA
; TEST TO SEE IF THE PIN WENT HIGH
ANDA #%00010000
BNE WENTHIGH
; NO, PIN WENT LOW. LOAD D WITH OFFTIME
LDD OFFTIME
BRA REST
WENTHIGH
; PIN WENT HIGH. LOAD D WITH ONTIME
LDD ONTIME
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