CMX850L8 View Datasheet(PDF) - CML Microsystems Plc
Part Name
Description
MFG CO.
'CMX850L8' PDF : 103 Pages View PDF
Communications Controller
CMX850
1.4.1 Ring Detector Interface
Figure 3 shows how the Modem 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 CMX850 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 DVSS by discharging the external capacitor C22. The output of the Schmitt trigger 'B' will
then go high, setting bit 14 (Ring Detect) of the modem’s Status Register 1.
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 Figure 16.
With R20 - R22 all 470kΩ as Figure 3, then setting R23 to 68kΩ will guarantee detection of ringing signals
of 40Vrms and above for DVDD over the range 3V to 3.6V.
C20 R20
2-Wire
Telephone
Line
D1 - 4
DVSS
C21 R21
X
R22 RD
R23
DVSS
CMX850
B
A
RT
C22
To Status
Register
R24
DVDD
Ring signal
Bridge rectifier o/p (X)
RT
Status Register bit 14
(Ring Detect)
Vthi
DVSS
Vthi
DVSS
R20, 21, 22
R23
R24
470kΩ
See text
470kΩ
C20, 21
C22
D1-4
0.1µF
0.33µF
1N4004
Resistors ±5%, capacitors ±20%
Figure 3 Ring Signal Detector Interface Circuit
© 2003 CML Microsystems Plc
12
D/850/6
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