A 4.7k PULLUP RESISTOR MUST BE CONNECTED
   BETWEEN DATA (PA6) AND PORTA HEADER PIN 10 (VTG) !

TERM:CLR TR0
JB DATA,$
MOV TH0,#HI(R_DEL)
MOV TL0,#LO(R_DEL)
SETB TR0
WAIT:JNB DATA,$
SJMP TERM

RES:      JNB DATA,$

MOV R3,#14
DJNZ R3,$;delay 30 uS
CLR DATA;presence pulse begin

MOV R3,#58
DJNZ R3,$;delay 120 uS
SETB DATA;presence pulse end

MOV R0,#FAM_CODE-1;pointer load
MOV R1,#9;load number of bytes (1 RX + 8 TX)
MOV R2,#8;load number of bits
SETB RXF;set RX flag
MOV A,#0FFH;perform RX
RRC A;C <- first TX bit
CLR IE0;clear interrupt flag

;1-Wire RX-TX Loop:

LOOPH:JNB IE0,$;wait for interrupt flag

MOV DATA,C;DATA <- C
MOV TH0,#HI(R_DEL)
MOV TL0,#LO(R_DEL)
SETB TR0;start timer 0 (reset timeout)
MOV R3,#9
DJNZ R3,$;delay
MOV C,DATA;receive data bit
SETB DATA;DATA <- 1 (30uS is over)
RRC A;save received bit in ACC

CLR IE0;clear interrupt flag
DJNZ R2,LOOP;advance bit counter
JNB RXF,NEXT;jump if TX cycle

CLR RXF
CJNE A,#033H,MISM;RX complete, check command code
SJMP NEXT
MISM:CJNE A,#00FH,TERM

NEXT:INC R0;advance pointer
MOV R2,#8;bits counter reload
MOV A,@R0
RRC A
DJNZ R1,LOOP
SJMP TERM

LOOP:     JNB DATA,$;wait for DATA = 1
CLR TR0
SJMP LOOPH

;TIMER0 Interrupt Holder:
;Timer terminates process if reset pulse detected

TIMER:CLR TR0;timer 0 stop
MOV SP,#STACK
MOV A,#LO(RES)
PUSH ACC
MOV A,#HI(RES)
PUSH ACC
RETI;return to RES

.END