CIRCUIT DESCRIPTION

After detecting presence of a valid DTMF signal StD goes high for Short duration (Refer Data sheet) and outputs Q1-Q4 gets latched according to received tone-pair  (Refer above table)

ALGORITHM

·         Poll for Ring  Detection.

·         Activate of relay to place 220 Ohm (1/2 watt.) resistor in line so as to lift phone .(The 220-ohm loop on telephone line disconnects the  ringer from the telephone line in the exchange.)

·          Poll for StD and when it goes high Read port 0  to find out received tone-pair from Q1-Q4.

·         Take control action according to received tone & Poll for the strobe again if you wish and  deactivate relay when you finish.

                COADING(ASSEMBLY LANGUAGE)  

·         mov p0 ,#0ffh      ; To configure P0 for input

·         setb p2.6             ; To configure P2.6 (RING DETECT) for input.

·         AGAIN

·         jnb p2.6 ,  AGAIN   ; Poll for ring detect signal

·         setb 2.7                 ; Activate relay to lift receiver

·         setb p0.3

·          

·         POLL

·         jnb p0.3 POLL       ;Poll for high strobe 

·         mov a,p0              ;Read P0

·         anl a,#0f0h          ;To get code (Q1-Q4 ) for Received tone-                              pair in ACC.

·         xrl a,#01h

·         jnz TERMINATE      ;If 0 is not received TERMINATE Session

·         setb p1.0              ;make P1.0 High if 1 is received

·         TERMINATE

·         clr 2.7                 ;Deactivate relay 

Presented By- SANJEEV SHARMA 

  [ Studying in EC 2nd Year]

  Naraina College Of Engineering & Technology, kanpur