FSK Modem and DTMF Codec
CMX865
If the time constant of R24 and C22 is large enough, then the voltage on RT will remain below the
threshold of the 'B' Schmitt trigger for the duration of a ringing cycle.
The time for the voltage on RT to charge from VSS towards VDD can be derived from the formula
V
= VDD x [1 - exp(-t/(R24 x C22)) ]
RT
As the Schmitt trigger high-going input threshold voltage (Vthi) has a minimum value of 0.56 x VDD, then
the Schmitt trigger B output will remain high for a time of at least 0.821 x R24 x C22 following a pulse at
RD.
The values of R24 and C22 given in Figure 3 (470kΩ and 0.33µF) give a minimum RT charge time of 100
msec, which is adequate for ringing frequencies of 10Hz or above.
Note that the circuit will also respond to a telephone line voltage reversal. If necessary the µC can
distinguish between a ringing signal and a line voltage reversal by measuring the time that b14 of the
Status Register (Ring Detect) is high.
If the Ring Detect function is not used then pin RD should be connected to VSS and RT to VDD.
4.2
Line Interface
A line interface circuit is needed to provide dc isolation and to terminate the line. Typical interface circuits
are described below.
2-Wire Line Interface
Figure 4a shows a simplified interface for use with a 600Ω 2-wire line. The complex line termination is
provided by R13 and C10, high frequency noise is attenuated by C10 and C11, while R11 and R12 set
the receive signal level into the modem. For clarity the 2-wire line protection circuits have not been shown.
R11
R12
R13
See text
100kΩ
600Ω
C3
C10
C11
See Figure 2
33nF
100pF
Resistors ±5%, capacitors ±20%
Figure 4a 2-Wire Line Interface Circuit
© 2005 CML Microsystems Plc
7
D/865/3
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