CMX868 查看數據表（PDF） - CML Microcircuits

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CMX868

Low Power V.22 bis Modem

CML Microcircuits 

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

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 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

100kΩ

See text

C10

33nF

C11

100pF

C12

0.1µF

Figuire 4a 2-Wire Line Interface Circuit

© 2004 CML Microsystems Plc

7

D/868/9

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