CMX867(2000) Просмотр технического описания (PDF) - CML Microsystems Plc
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CMX867 Datasheet PDF : 45 Pages
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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.
2-Wire Line Interface
Figure 4a shows an 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
The transmit line signal level is determined by the voltage swing between the TXA and TXAN pins, less 6dB
due to the line termination resistor R13, and less the loss in the line coupling transformer.
© 2000 Consumer Microcircuits Limited
7
D/867/1
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