MT88E45
MT88E45
Advance Information
Vdd
Both Tones Present
P Q1
+
-
Comparator
R1
N Q2
= Vss
EST
Rp=R1 || R2
DR
/
STD
Indicates STD in CAS
detection mode
CAS
t
DP
EST
t
GP
ST/GT
t
REC
t
GA
t
DA
t
ABS
t
GP
=R1C ln [Vdd / (Vdd-VTGt)]
t
GA
=RpC ln
Rp=R1 || R2
t
GA
=0 if R2=0
STD
Vdd - Vdiode (Rp/R2)
VTGt - Vdiode (Rp/R2)
R2
VTGt
C
ST/GT
V
diode
Figure 5 - CAS Guard Time Circuit Operation
dropouts once EST has met the minimum tone
duration so as to decrease the likelihood of a long
talkoff being broken up into several talkoffs. Usually
t
GA
is set very short or removed altogether because
there is another way to deal with the problem (by
ignoring further detections for 2 seconds after every
detection).
Tone present guard time (t
GP
) operation: In Figure 5
initially there is no CAS, EST is low so Q1 is off. C
has been fully charged applying 0V to ST/GT so Q2
is on. When both tones are detected EST goes high
and turns off Q2. Because C has been fully charged
(ST/GT=0V), the comparator output is low and Q1
stays off. With both Q1 and Q2 off the high at EST
discharges C through R1 and the ST/GT voltage
increases from 0V. When the voltage exceeds the
comparator threshold VTGt, which is typically 0.5
Vdd, the comparator output goes high; Q1 turns on
and accelerates the discharge of C (ST/GT goes
quickly to Vdd); STD goes low to indicate that a valid
CAS has been received. If one or both tones
disappeared before t
GP
has been reached (i.e. when
ST/GT voltage is still below VTGt), Q2 turns back on
8
and charges C quickly to bring the ST/GT voltage
back to 0V. Then if EST goes high again the t
GP
duration must start over.
Tone absent guard time (t
GA
) operation: In Figure 5
initially both tones have been detected for t
GP
so C is
fully discharged and ST/GT is at Vdd. While both
tones continue to be detected EST stays high; ST/GT
is at Vdd (the comparator output is high); so Q1 is on
and Q2 is off. When one or both tones stop EST
goes low and turns off Q1. Because C is fully
discharged (ST/GT=Vdd), the comparator output is
high and Q2 stays off. With both Q1 and Q2 off the
low at EST charges C through Rp=(R1 || R2) and the
ST/GT voltage falls towards 0V. When the voltage
has fallen below VTGt, the comparator output goes
low. Since EST is also low Q2 turns on and
accelerates the charging of C so that ST/GT goes
quickly to 0V. STD goes high to indicate that the CAS
has ended. If EST goes back to high before t
GA
has
been reached (i.e. when ST/GT voltage is still above
V
TGt
), Q1 turns back on and discharges C quickly to
bring the ST/GT voltage back to Vdd. Then if EST
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