mt88ebnr Zarlink Semiconductor, mt88ebnr Datasheet - Page 10

• Manufacturer Part Number: mt88ebnr
• Description: 4-wire Calling Number Identification Circuit 2 4-wire Cnic2 Semiconductor Inc
• Manufacturer: Zarlink Semiconductor

Tone present guard time (t
fully charged applying 0 V 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 0 V. 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
VTGt), Q2 turns back on and charges C quickly to bring the ST/GT voltage back to 0 V. Then if EST goes high again
the t
Tone absent guard time (t
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
discharges C quickly to bring the ST/GT voltage back to Vdd. Then if EST goes low again the t
over. To set t
GP
CAS
EST
ST/GT
STD
duration must start over.
Both Tones Present
GA
=0, set R2 to 0.
GA
has been reached (i.e. when ST/GT voltage is still above V
GP
GA
t
DP
) operation: In Figure 5 5 initially there is no CAS, EST is low so Q1 is off. C has been
) operation: In Figure 5 5 initially both tones have been detected for t
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Figure 5 - CAS Guard Time Circuit Operation
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t
REC
Zarlink Semiconductor Inc.
Comparator
MT88E45
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10
GP
t
DA
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has been reached (i.e. when ST/GT voltage is still below
t
GA
VTGt
t
ABS
P
N
Q2
Q1
= Vss
MT88E45B
t
t
Rp=R1 || R2
t
GP
GA
GA
=R1C ln [Vdd / (Vdd-VTGt)]
=RpC ln
=0 if R2=0
TGt
ST/GT
EST
Vdd
DR
Indicates STD in CAS
detection mode
), Q1 turns back on and
R1
VTGt - Vdiode (Rp/R2)
Vdd - Vdiode (Rp/R2)
/
STD
GA
duration must start
Rp=R1 || R2
GP
C
Data Sheet
R2
V
so C is fully
diode