HT9170 datasheet(7/12 Pages) HOLTEK

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HT9170

December 20, 1999

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).

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.

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

VR EF

H T 9170

Se r i e s

( a ) S t andar d i n put c i r c ui t

VR EF

H T 9170

Se r i e s

(b ) D i f f e re n t i a l i n p ut c i r c ui t

V i1

V i2

V i

Figure 2. Input operation for amplifier applica-

tion circuits

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