' **** "BASIC STAMP' demo for the ROHM BU8874 touch tone DTMF decoder ****
' ****          (C) 3/18/93 Jim Gillespie  Version 1.5 ***
' ****                      1061 Elizabeth St. #207
' ****			    Denver, Co 80206

' ** port direction definitions
input 	0 	' tone present true high ( ROHM chip ESt pin 7 )
output  1   	' data clock true high   ( ROHM chip ACK pin 6 )
input	2   	' touch tone data	 ( ROHM chip SD pin 5  )
output	3	' relay and lamp driver true high ( BS170 TMOS transistor )

' ** output port initialization **
low 	1	' clock low
low	3	' relay and lamp off

' ** register file variable initialization
W4=0		' 4 DTMF values assembled here bas ackwards
w5=$21CB	' DTMF  * # 1 2  match values bas ackwards ( saves time )

' **  somewhat of a software state machine follows
state1:	if pin0=1 then state1   ' if tone still present, wait ( ESt )
state2:	if pin0=0 then state2   ' hang here while no tone ( ESt ) present
state3:	if pin0=0 then state1   ' if a fairly quick 1 to 0, its a "glitch" 

	w3=w4			' save old value in case of a BCD "glitch"

	for b3=1 to 4		' 4 bits to assemble
           W4=W4 / 2		' shift last BCD value right 1 bit
	   high 1		' clock high ( ACK. SD data will appear )
	   if pin2 = 0 then state4 ' read SD bit. if 0 is read, do nothing
	   b9 = b9 | $80        ' SD a 1. put in the msb of BCD word
state4:	   low 1		' clock low ( ACK )
	next b3			' loop til 4 bits assembled

	if pin0=0 then statex	' tone detect gone. probably a "glitch"
	if w4 <> w5 then state1	' did 4 BCD digits match?
	toggle 3		' YES! toggle relay and LED TMOS transistor
	goto state1		' we got a BCD match & no "glitch". loop. 
 
statex:	w4=w3			' probably a "glitch". restore previous
	goto state1		' BCD value and try again.

	end