Calling Line Identifier
MX602 - PRELIMINARY INFORMATION
11
A line polarity reversal may be detected using the same circuit but there will be only one pulse at RD. The British Telecom
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
t
−
dV
dt
dV
dt
T
C3 1− e
R2 where T = C3 R1+ R2 + R3 and
is the input slew rate.
(
)
For dV/dt = 500V/sec (15V in 30msec), R1, R3 and R4 all 470kΩ and C3, C4 both 0.1µF as indicated in Figure 2, then
setting R2 to 390kΩ will guarantee detection at V = 5.5V.
DD
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 IRQ outputs active 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
t
−
R5C5
V
= V
1− e
RT
DD
As the Schmitt trigger high-going input threshold voltage (Vt ) has a minimum value of 0.56 x V , then the Schmitt
HI
DD
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(470kΩ and 0.33µF) gives a minimum time of 100 msec (independent of V ), which is
DD
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 IRQ or DET output.
4.9 Xtal Osc and Clock Dividers
Frequency and timing accuracy of the MX602 is determined by a 3.579545MHz clock present at the XTAL pin. This may
be generated by the on-chip oscillator inverter using the external components C1, C2 and X1 of Figure 2, or may be
supplied from an external source to the XTAL input, in which case C1, C2 and X1 should not be fitted.
The oscillator is turned off in the 'Zero-Power' modes.
If the clock is provided by an external source which is not always running, then the ZP input must be set high when the
clock is not available. Failure to observe this rule may cause a significant rise in the supply current drawn by MX602 as
well as generating undefined states of the RXD, DET and IRQ outputs.
© 1998 MX•COM Inc.
www.mxcom.com Tele: 800 638-5577 336 744-5050 Fax: 336 744-5054
Doc. # 20480136.004
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