HT9170B(1999) 데이터 시트보기 (PDF) - Holtek Semiconductor
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| HT9170B Datasheet PDF : 0 Pages | |||
Functional Description
Overview
The HT9170 series tone decoders consist of
three band pass filters and two digital decode
circuits to convert a tone (DTMF) signal into
digital code output.
An operational amplifier is built-in to adjust
the input signal (refer to Figure 2).
VP
C
R1
Vi
VN
H T9170
RF
GS
S e r ie s
VREF
( a ) S ta n d a r d in p u t c ir c u it
C
R1
V i1
VP
V i2
VN
C
R2
R3 R4 R5
H T9170
GS
S e r ie s
VREF
( b ) D iffe r e n tia l in p u t c ir c u it
Figure 2. Input operation for amplifier applica-
tion circuits
The pre-filter is a band rejection filter which re-
duces the dialing tone from 350Hz to 400Hz.
The low group filter filters low group frequency
signal output whereas the high group filter fil-
ters high group frequency signal output.
Each filter output is followed by a zero-crossing
detector with hysteresis. When each signal am-
plitude at the output exceeds the specified
level, it is transferred to full swing logic signal.
When input signals are recognized to be effec-
tive, DV becomes high, and the correct tone
code (DTMF) digit is transferred.
HT9170
Steering control circuit
The steering control circuit is used for measur-
ing the effective signal duration and for protect-
ing against drop out of valid signals. It employs
the analog delay by external RC time-constant
controlled by EST.
The timing is shown in Figure 3. The EST pin is
normally low and draws the RT/GT pin to keep
low through discharge of external RC. When a
valid tone input is detected, EST goes high to
charge RT/GT through RC.
When the voltage of RT/GT changes from 0 to
VTRT (2.35V for 5V supply), the input signal is
effective, and the correct code will be created by
the code detector. After D0~D3 are completely
latched, DV output becomes high. When the
voltage of RT/GT falls down from VDD to VTRT
(i.e.., when there is no input tone), DV output
becomes low, and D0~D3 keeps data until a
next valid tone input is produced.
By selecting adequate external RC value, the min-
imum acceptable input tone duration (tACC) and
the minimum acceptable inter-tone rejection (tIR)
can be set. External components (R, C) are chosen
by the formula (refer to Figure 5.):
tACC=tDP+tGTP;
tIR=tDA+tGTA;
where tACC: Tone duration acceptable time
tDP: EST output delay time (²L²®²H²)
tGTP: Tone present time
tIR: Inter-digit pause rejection time
tDA: EST output delay time (²H²®²L²)
tGTA: Tone absent time
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December 20, 1999
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