Low Power V.22 bis Modem
CMX868
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 ring 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 (470kW and 0.33mF) give a minimum RT charge time of
100 msec, which is adequate for ring frequencies of 10Hz or above.
Note that the circuit will also respond to a telephone line voltage reversal. If necessary the mC can
distinguish between a Ring signal and a line voltage reversal by measuring the time that bit 14 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.
1.4.2 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 an interface circuit for use with a 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,R14,R15
See text
100kW
See text
C10
C11
C12
33nF
100pF
0.1µF
Figuire 4a 2-Wire Line Interface Circuit
ã 2004 CML Microsystems Plc
7
D/868/9
CMLMICRO [ CML MICROCIRCUITS ]
