CMX624 데이터 시트보기 (PDF) - MX-COM Inc

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CMX624

Bell 202 and V.23 Modem with Call Progress and DTMF

MX-COM Inc  

Bell 202 and V.23 Compatible Modem

18

CMX624 Preliminary Information

Note:

1. 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

1

-

e

−t

R 24 xC22









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 14 (470kΩ and 0.33µF) give a minimum RT charge time of

100ms, which is adequate for ring frequencies of 10Hz or above.

Note: 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.

©1999 MX-COM, Inc.

www.mxcom.com Tel: 800 638 5577 336 744 5050 Fax: 336 744 5054

Doc. # 20480180.108

4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA

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