HT9170
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).
C
V
i
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
V
TRT
(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 V
TRT
(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 (t
ACC
) and
the minimum acceptable inter-tone rejection (t
IR
)
can be set. External components (R, C) are chosen
by the formula (refer to Figure 5.):
t
ACC
=t
DP
+t
GTP
;
t
IR
=t
DA
+t
GTA
;
where t
ACC
: Tone duration acceptable time
t
DP
: EST output delay time (²L²®²H²)
t
GTP
: Tone present time
t
IR
: Inter-digit pause rejection time
t
DA
: EST output delay time (²H²®²L²)
t
GTA
: Tone absent time
R 1
V P
V N
R F
G S
V R E F
H T 9 1 7 0
S e r ie s
( a ) S ta n d a r d in p u t c ir c u it
V
V
C
i1
i2
R 1
V P
V N
C
R 2
R 3
R 4
R 5
G S
V R E F
H T 9 1 7 0
S e r ie s
( 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.
7
December 20, 1999
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
