V.22 bis Modem with AT Commands
CMX866
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 DVSS towards DVDD can be derived from the formula:
V
= DVDD 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 DVDD
,
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 100msec, which is adequate for ring frequencies of 10Hz or above.
Note that this circuit will also respond to a telephone line voltage reversal. The external host mC can
distinguish between a Ring signal and a line voltage reversal by measuring the time that pin RIN is low.
If the Ring detect function is not used then pin RD should be connected to DVSS and RT to DVDD.
1.4.2 Line Interface
A line interface circuit is needed to provide dc isolation and to terminate the line.
2-Wire Line Interface
Figure 4a shows a simplified interface for use with a 600W 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 and the connection
to RXBN (for on-hook CLI applications) have not been shown.
R11
R12
R13
See text
100kW
600W
C3
C10
C11
See Figure 2
33nF
100pF
Resistors ±5%, capacitors ±20%
Figure 4a 2-Wire Line Interface Circuit
ã 2004 CML Microsystems Plc
9
D/866/4
CMLMICRO [ CML MICROCIRCUITS ]
