CMX867D2 CML Microcircuits, CMX867D2 Datasheet - Page 7

• Manufacturer Part Number: CMX867D2
• Description: Low power V.22 modem
• Manufacturer: CML Microcircuits

Low Power V.22 Modem
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 V
As the Schmitt trigger high-going input threshold voltage (Vthi) has a minimum value of 0.56 x V
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 (470k
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 C 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 V
1.4.2
A line interface circuit is needed to provide dc isolation and to terminate the line.
2-Wire Line Interface
Figure 4a shows an interface for use with a 600
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.
2001 Consumer Microcircuits Limited
Line Interface
R11
R12
R13
See text
100k
600
Figure 4a 2-Wire Line Interface Circuit
V RT = V
Resistors 5%, capacitors 20%
DD
x [1 - exp(-t/(R24 x C22)) ]
SS
2-wire line. The complex line termination is provided by
towards V
7
and 0.33 F) give a minimum RT charge time of
C3
C10
C11
DD
can be derived from the formula
See Figure 2
33nF
100pF
SS
and RT to V
DD
.
DD
CMX867
D/867/2
, then