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Page 7




2. Telephone Interface and Number Encoding


MT 8870 forms the primary interface to the system by providing tolerance to small frequency deviations and variations in the telephone line (Fig. 2). This ensures an optimum combination of immunity to talk-off and tolerance to the presence of interfering frequencies (third tones) and noise. When the detector recognises the presence of two valid tones ESt output will go to an active state and any subsequent loss of signal condition will cause ESt to assume an inactive state. This information is relayed through PA5 of PIO to Z80 for number registration. Before registration of a decoded tone pair, the DTMF receiver checks for a valid signal duration. This check is performed by R12C5 time constant driven by ESt. A logic high on ESt causes Vc ( Fig. 2) to rise as the capacitor C5 discharges. Provided signal condition is maintained for the validation period (t GTP ), and V c reaches the threshold  (VTSt ) of the steering logic to register the tone pair, latching its corresponding 4-bit code (see Table 1) into the output latch. At this point, the GT output is activated and drives Vc to V DD. GT continues to drive high as long as ESt remains high. Finally, after a short delay to allow the output latch to settle, the delayed steering output flag (StD) goes high, signalling that a received tone pair has been registered. The contents of the output latch are made available on the 4-bit output bus to Port A of PIO by raising the three state control input (TOE) to a logic high. The steering circuit works in reverse to validate the interdigit pause between signals. Thus, as well as rejecting signals too short to be considered valid, the receiver will tolerate signal interruptions too short to be considered a valid pause. The internal clock circuit is completed with the addition of an external 3.579545 MHz crystal which is also common for Z80.

V TSt  = 0.465V DD  (the nominal approximation provided by manufacturer)

t GTA =(R12C5 )In(V DD /V TSt )  = 2.74 ms  (the guard times for tone absent)

         t GTP =(R12C5)In[V DD /(V DD -V TSt )] = 1.03 ms (the             guard times for tone present)


Fig.2 DTMF number encoding circuit and telephone interfacing to ITDL



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Last modified: July 13, 2002