SC88E43 View Datasheet(PDF) - Silan Microelectronics
Part Name
Description
Manufacturer
SC88E43 Datasheet PDF : 25 Pages
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Silan
Semiconductors
SC88E43
In the SC88E43 the dual tone signal is separated into a high and a low tone by two bandpass filters. A detection
algorithm examines the two filter outputs to determine the presence of a dual tone alert signal. The ESt pin goes
high when both tones are present. Note that ESt is only a preliminary indication. The indication must be sustained
over the tone present guard time to be considered valid. Tone present and tone absent guard times can be
implemented with external RC components. The tone present guard time rejects signals of insufficient duration.
The tone absent guard time masks momentary detection dropout once the tone present guard time has been
satisfied. StD is the guard time qualified detector output.
Dual Tone Detection Guard Time
When the dual tone signal is detected by the SC88E43, ESt goes high. When the signal ceases to be detected,
ESt goes low.
The ESt pin indicates raw detection of the dual tone signal. Since the BT application requires a minimum signal
duration and the Bellcore application requires protection from imitation by speech, Est detection must be guard
time qualified. The StD pin provides guard time qualified signal detection. When the SC88E43 is used in a caller
identity system, StD indicates correct CAS/Tone Alert Signal detection.
Tone detected
SC88E43
VDD
Figure 4 : Guard Time Circuit Operation
From
detector
P Q1
C
+
-
VTGt
Comparator
N Q2
=VSS
St/GT
A
ESt
R
SW1
B
VSS
StD
Figure 4 shows the relationship between the
St/GT, ESt and StD pins. It also shows the
operation of the guard time circuit.
The total recognition time is tREC = tGP + tDP , where tGP is the tone present guard time and tDP is the tone
present detect time (refer to timing between ESt, St/GT and StD in Figures 17 and 20).
The total tone absent time is tABS = tGA + tDA , where tGA is the tone absent guard time and tDA is the tone
absent detect time (refer to timing between ESt, St/GT and StD in Figures 17 and 20).
Bellcore states that it is desirable to be able to turn off CAS detection for an off-hook capable CPE. The disable
switch allows the subscriber who disconnects a service that relies on CAS detection (e.g., CIDCW) but retains the
CPE, to turn off the detector and not be bothered by false detection.
When SW1 in Figure 4 is in the B position the guard time circuit is disabled. The detector will still process
CAS/Alerting tones but the SC88E43 will not signal their presence by ensuring that StD is low.
BT specifies that the idle state tone alert signal recognition time should not be less than 20ms when both tones are
used for detection. That is, both tones must be detected together for at least 20ms before the signal can be
declared valid. This requirement can be met by setting the tGP (refer to Figure 5) to at least 20ms.
HANGZHOU SILAN MICROELECTRONICS JOINT-STOCK CO.,LTD
Rev: 1.0 2000.12.31
11
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