MT88E45
Advance Information
MT88E45
Vdd
Both Tones Present
P
Q1
C
+
ST/GT
-
VTGt
V
diode
Comparator
R1
R2
Q2
N
= Vss
EST
Rp=R1 || R2
DR/STD
Indicates STD in CAS
detection mode
CAS
EST
t
t
DA
DP
t
ABS
t
t
=R1C ln [Vdd / (Vdd-VTGt)]
GP
t
REC
=RpC ln Vdd - Vdiode (Rp/R2)
VTGt - Vdiode (Rp/R2)
GA
t
t
GA
GP
Rp=R1 || R2
=0 if R2=0
ST/GT
STD
t
GA
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
tGA 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).
and charges C quickly to bring the ST/GT voltage
back to 0V. Then if EST goes high again the tGP
duration must start over.
Tone absent guard time (tGA) operation: In Figure 5
initially both tones have been detected for tGP 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 tGA has
been reached (i.e. when ST/GT voltage is still above
VTGt), Q1 turns back on and discharges C quickly to
bring the ST/GT voltage back to Vdd. Then if EST
Tone present guard time (tGP) 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 tGP has been reached (i.e. when
ST/GT voltage is still below VTGt), Q2 turns back on
8
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