Telephone Signalling Transceiver / Least Cost Router
CMX860
If the time constant of R24 and C22 is large enough then the voltage on RT will remain below the
threshold of the 'B' Schmitt trigger for the duration of a ring cycle.
The time for the voltage on RT to charge from DV
SS
towards DV
DD
can be derived from the formula:
VRT = DV
DD
x [1 - exp(-t/(R24 x C22)) ]
As the Schmitt trigger high-going input threshold voltage (Vthi) has a minimum value of 0.56 x DV
DD
,
then the Schmitt trigger B output will remain high for a time of at least 0.821 x R24 x C22 following a
pulse at RD.
The values of R24 and C22 given in Figure 3 (470kΩ and 0.33µF) give a minimum RT charge time of
100 msec, which is adequate for ring frequencies of 10Hz or above.
Note that the circuit will also respond to a telephone line voltage reversal. If necessary the
µC
can
distinguish between a Ring signal and a line voltage reversal by measuring the time that bit 14 of the
Status Register (Ring Detect) is high.
If the Ring detect function is not used then pin RD should be connected to DV
SS
and RT to DV
DD
.
1.4.2
Hook Detector Interface
This is identical internally to the Ring Detector interface circuit and similar components could be used
externally, with appropriate values, if hook detection is to be performed by detecting a voltage change
across the tip and ring lines to the local phone.
1.4.3
RESETN pin
When this pin is taken low, it performs the same operation as a C-BUS General Reset command. As a
consequence the device will enter the powersaved state. Refer to section 1.5.12.1 (General Reset
Command) and 1.5.12.2 (General Control Register, Powerup bit) for further information.
©
2002 Consumer Microcircuits Limited
11
D/860/5
CMLMICRO [ CML MICROCIRCUITS ]
