V.22 bis Modem with AT Commands
CMX866
©
2008 CML Microsystems Plc
8
D/866/5
1.4.1
Ring Detector Interface
Figure 3 shows how the CMX866 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 CMX866 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 DV
SS
by discharging the external capacitor C22. The output of the Schmitt trigger 'B' will
then go high, setting bit 14 (Ring Detect) of the DSP Status Register. The on-chip µController will then
respond by setting pin RIN low.
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 470k
Ω
as Figure 3, setting R23 to 68k
Ω
will guarantee detection of ringing signals of
40Vrms and above for DV
DD
over the range 3 to 5V.
R20, 21, 22
470k
Ω
C20, 21
0.1
μ
F
R23
See
text
C22
0.33
μ
F
R24
470k
Ω
D1-4 1N4004
Resistors
±
5%, capacitors
±
20%
Figure 3 Ring Signal Detector Interface Circuit