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
