CMX602A 查看數據表（PDF） - MX-COM Inc
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CMX602A Datasheet PDF : 21 Pages
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Calling Line Identifier/Calling Line Identifier on Call Waiting 11
CMX602A PRELIMINARY INFORMATION
When no signal is present on the telephone line, RD will be at VSS and RT pulled to VDD 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. The ring signal is 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 in a rectified and attenuated form.
The signal prior to R1 will be further attenuated by the potential divider formed by R1 and R2 before being
applied to the CMX602A 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 VSS by discharging the external capacitor C5. The output of the Schmitt trigger 'B' will then
go high, activating the DET and/or IRQ 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
R1
R2
R2
R3 ·
¸
¹
0.707VRMS
Where VtHI is the high-going threshold voltage of the Schmitt trigger A (see Section 6.1).
With R1, R3 and R4 all 470k: as indicated in Figure 2, then setting R2 to 68k: will guarantee detection of
ringing signals of 40VRMS and above for VDD = 2.7 to 5.5V.
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 indicates 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
dV
dt
§
C3©¨¨1
e
t·
T
¸
¸
R2
¹
where T C3 R1 R2 R3 and dV is the input slew rate.
dt
For dV/dt = 500V/sec (15V in 30ms), R1, R3 and R4 all 470k: and C3, C4 both 0.1PF as indicated in Figure
2, then setting R2 to 390k: will guarantee detection at VDD = 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 IRQ outputs active for the duration of a ring cycle
The time for the voltage on RT to charge from VSS towards VDD can be derived from the formula
VRT
§
VDD ¨¨1
e
t
R5C5
·
¸
¸
©
¹
As the Schmitt trigger high-going input threshold voltage (VtHI) has a minimum value of 0.56 x VDD, 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 provided in Figure 2 (470k: and 0.33PF) gives a minimum time of 100ms (independent of
VDD), 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 IRQ or DET
output.
¤1999 MX-COM, Inc
www.mxcom.com Tel: 800 638 5577 336 744 5050 Fax: 336 744 5054
Doc. # 20480169.004
4800 Bethania Station Road, Winston-Salem, NC 27105-1201, USA All trademarks and service marks are held by their respective companies.
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