Calling Line Identifier
FX602
When no signal is present on the telephone line, RD will be at V
SS
and RT pulled to V
DD
by R5 so the 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 FX602 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
SS
by discharging the external capacitor C5. The output of the Schmitt trigger 'B' will 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
( 0.7 + Vthi x [R1 + R2 + R3] / R2 ) x 0.707 Vrms
where Vthi is the high-going threshold voltage of the Schmitt trigger A (see section 1.7).
With R1, R3 and R4 all 470k
Ω
as Figure 2, then setting R2 to 68k
Ω
will guarantee detection of ringing signals
of 40Vrms and above for V
DD
over the range 3.0 to 5.5V.
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
30msec. For a linearly changing voltage at the input to C3 (or C4), then the voltage appearing at the RD pin will
be
dV/dt x C3 x [ 1 - exp(-t/T) ] x R2
where T = C3 x (R1 + R2 + R3) and dV/dt is the input slew rate.
For dV/dt = 500V/sec (15V in 30msec), R1, R3 and R4 all 470k
Ω
and C3, C4 both 0.1
µ
F as Figure 2, then
setting R2 to 390k
Ω
will guarantee detection at V
DD
= 5.5V.
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
SS
towards V
DD
can be derived from the formula
VRT = V
DD
x [1 - exp(-t/(R5 x C5)) ]
As the Schmitt trigger high-going input threshold voltage (Vthi) has a minimum value of 0.56 x V
DD
, then 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
Ω
and 0.33
µ
F) gives a minimum time of 100 msec (independent of V
DD
), which is adequate for ring frequencies of 10Hz or above.
If necessary, the µC can distinguish between a ring and a reversal by timing the length of the IRQN or DET
output.
©
1998 Consumer Microcircuits Limited
12
D/602/7
CMLMICRO [ CML MICROCIRCUITS ]
