CMX624 Просмотр технического описания (PDF) - CML Microsystems Plc

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CMX624

V.23 / Bell 202 Modem

CML Microsystems Plc 

V23 / Bell 202 Modem

CMX624

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 9 (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 6 of the

FLAGS Register (Ring Detect) is high.

 2003 CML Microsystems Plc

18

D/624/7

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