CMX602B Consumer Microcircuits Limited, CMX602B Datasheet - Page 12

• Manufacturer Part Number: CMX602B
• Description: Calling Line Identifier Plus Call Waiting Type ii
• Manufacturer: Consumer Microcircuits Limited

Calling Line Identifier
When no signal is present on the telephone line, RD will be at V
output of the Schmitt trigger 'B' will be low.
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 C3 and R3 or C4 and R4 to appear at the top end of R1
(point X in Figure 7) in a rectified and attenuated form.
The signal at point X will be further attenuated by the potential divider formed by R1 and R2 before being
applied to the CMX602B input RD . 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 V
then go high, activating the DET and/or IRQN outputs depending on the states of the MODE and ZP
inputs.
The minimum amplitude ringing signal that is certain to be detected is
where Vthi is the high-going threshold voltage of the Schmitt trigger A (see section 1.7).
With R1, R3 and R4 all 470k
signals of 40Vrms and above for V
A line polarity reversal may be detected using the same circuit but there will be only one pulse at RD. The
BT specification SIN242 says that the circuit must detect a +15V to -15V reversal between the two lines
slewing in 30ms. For a linearly changing voltage at the input to C3 (or C4), then the voltage appearing at
the RD pin will be
where T = C3 x (R1 + R2 + R3) and dV/dt is the input slew rate.
For dV/dt = 500V/sec (15V in 30ms), R1, R3 and R4 all 470k
setting R2 to 390k
If the time constant of R5 and C5 is large enough then the voltage on RT will remain below the threshold of
the 'B' Schmitt trigger keeping the DET and/or IRQN outputs active for the duration of a ring cycle
The time for the voltage on RT to charge from V
As the Schmitt trigger high-going input threshold voltage (Vthi) has a minimum value of 0.56 x V
the Schmitt trigger B output will remain high for a time of at least 0.821 x R5 x C5 following a pulse at RD.
Using the values given in Figure 2 (470k
V
If necessary, the µC can distinguish between a ring and a reversal by timing the length of the IRQN or DET
output.
 2003 CML Microsystems Plc
DD
), which is adequate for ring frequencies of 10Hz or above.
SS
will guarantee detection at V
by discharging the external capacitor C5. The output of the Schmitt trigger 'B' will
( 0.7 + Vthi x [R1 + R2 + R3] / R2 ) x 0.707 Vrms
as Figure 2, then setting R2 to 68k
DD
V RT = V
dV/dt x C3 x [ 1 - exp(-t/T) ] x R2
over the range 2.7 to 5.5V.
and 0.33 F) gives a minimum time of 100 ms (independent of
DD
x [1 - exp(-t/(R5 x C5)) ]
SS
DD
towards V
12
= 5.5V.
DD
can be derived from the formula
and C3, C4 both 0.1 F as Figure 2, then
SS
and RT pulled to V
will guarantee detection of ringing
DD
by R5 so the
DD
CMX602B
D/602B/2
, then