CMX867 查看數據表（PDF） - CML Microsystems Plc

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CMX867

Low Power V.22 Modem

CML Microsystems Plc 

Low Power V.22 Modem

CMX867

1.4.1 Ring Detector Interface

Figure 3 shows how the CMX867 may be used to detect the large amplitude Ringing signal voltage

present on the 2-wire line at the start of an incoming telephone call.

The ring signal is usually applied at the subscriber's exchange as an ac voltage inserted in series with

one of the telephone wires and will pass through either C20 and R20 or C21 and R21 to appear at the top

end of R22 (point X in Figure 3) in a rectified and attenuated form.

The signal at point X is further attenuated by the potential divider formed by R22 and R23 before being

applied to the CMX867 RD input. If the amplitude of the signal appearing at RD is greater than the input

threshold (Vthi) of Schmitt trigger 'A' then the N transistor connected to RT will be turned on, pulling the

voltage at RT to VSS by discharging the external capacitor C22. The output of the Schmitt trigger 'B' will

then go high, setting bit 14 (Ring Detect) of the Status Register.

The minimum amplitude ringing signal that is certain to be detected is:

( 0.7 + Vthi x [R20 + R22 + R23] / R23 ) x 0.707 Vrms

where Vthi is the high-going threshold voltage of the Schmitt trigger A (see section 1.7.1).

With R20-22 all 470kΩ as Figure 3, then setting R23 to 68kΩ will guarantee detection of ringing signals

of 40Vrms and above for VDD over the range 3 to 5V.

R20, 21, 22

R23

R24

470kΩ

See text

470kΩ

C20, 21

C22

D1-4

Resistors ±5%, capacitors ±20%

0.1µF

0.33µF

1N4004

Figure 3 Ring Signal Detector Interface Circuit

© 2004 Consumer Microcircuits Limited

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D/867/5

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