V23 / Bell 202 Modem
CMX624
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 V towards V
can be derived from the formula
SS
DD
V
= V x [1 - exp(-t/(R24 x C22)) ]
DD
RT
As the Schmitt trigger high-going input threshold voltage (Vthi) has a minimum value of 0.56 x V , then
DD
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 9 (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 6 of the
FLAGS Register (Ring Detect) is high.
2003 CML Microsystems Plc
18
D/624/7
CMLMICRO [ CML MICROCIRCUITS ]
