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CMX867D2

Low Power V.22 Modem

CML Microsystems Plc 

Low Power V.22 Modem

CMX867

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

VRT = VDD x [1 - exp(-t/(R24 x C22)) ]

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 ring 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 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 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

See text

R12

100kΩ

R13

600Ω

C3

See Figure 2

C10

33nF

C11

100pF

Resistors ±5%, capacitors ±20%

Figure 4a 2-Wire Line Interface Circuit

© 2004 Consumer Microcircuits Limited

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D/867/5

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