SGS Thomson Microelectronics ST3917B Datasheet

ST3917A
ST3917B

SPEECH - TONE/PULSE DIALER - LED INDICATOR

SPEECH CIRCUIT

.

2 TO4 WIRESCONVERSION

.

PRESENTTHEPROPERDC PATH FOR THE LINE
CURREN T AND THE FLEXIBILITY TO ADJUST IT
ANDALLOWPARALLELPH ONEOP ERATION

.

SYMMETRICAL HIGH IMPEDANCE MICRO­PHON E INPUTS SUIT ABLE FOR DYNAM IC ,
ELECTRET OR PIEZOELECTRIC TRANSDUCER

.

ASYMMETRICAL EARPHONE OUTPUT SUIT­ABLEFOR DYNAMIC TRANSDUCER

.

LINE LOSS COMPENSATION FIXED INTER­NALLYBYA.G.C.

.

INTERNAL MUTING TO DISABLE SPEECH
DURING DIALING AND EXTERNAL MUTING
TO DISABLE TRANSMIT AMPLIFIER DURING
CONVERSATIONMODE

.

LEDIND IC A T OREIT H E RFORKEYBOAR DILLU­MINATION (OR ON-LINE INDICATION) DURING
DIALING AND CONVERSATION OR FOR TONE
MODE INDICATION, THROUGH MU/MFI PIN

DIALER CIRCUIT

.

STOREUP TO 32 DIGITS FOR LAST NUMBER
REDIAL BUFFER, LNR IS INHIBITED IF THERE
AREMOR ETH AN32DIGITSSTORED

.

ALLOWMIXED MODE DIALINGIN PULSE MODE

.

PACIFIER TONE PROVIDES AUDIBLE INDI­CATIONOF VALID KEY PRESSEDIN A BUZZ­ER OR/AND IN THEEARPHONE

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TIMEDPABX PAUSE/ 10PPSPULSE RATE

.

MAKE/BREAK RATIO : 40/60 (ST3917A) AND
33/67(ST3917B)

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4 SELECTABLE OPTIONS ON FLASH DURA­TION AND SOFTSWITCH INHIBITION IN ONE
OF THE OPTIONWITH100msFLASHTIMING

.

2 SELECTABLE OPTIONS : TRANSMIT MUTE
TOGETHERWITHLEDFORKEYBOAR DILLU M I­NATIONOR LED FOR TONE MODE INDICATION

.

CONTINUOUS TONE FOR EACH DIGIT UNTIL
KEYRELEASE

.

USES INEXPENSIVE 3.579545MHz CERAMIC
RESONATOR

.

POWERED FROM TELEPHONE LINE, LOW
OPERATINGVOLTAGE FOR LONG LOOP AP­PLICATION

DESCRIPTION

Thedev iceconsis tsof thespeechand the dialerfunc­tion s .It providesthe DCline interfacecircuit thattermi ­nates the telephoneline, analog ampli f i erfor speech
transmis sion and necessary signals for either DTMF
and pulse dialing. When mated with a tone ringer, a
comp letetelepho necanbeproduc edwithjusttwoICs.

TheDClineinterfacecircuitdev elopsi t sow nlinevoltage
acro s sthedeviceanditisadjus t ablebyexternalres istor
to suit different countr y ’s speci f icatio n. A built - i n LED
drivercan deliverexcessline currentto externalLED
indic ator(s) duri ng dialing and speech mode. The
LED(s) can be used either for keyboard illumination
purposeor fortonemodeindication duringsoftswitch
andmixedmodediali ngby connec tingMU/MFI pi nto
VDDandGNDoranyrowr es pect ively.TheLEDcurrent
is limited to17mA(typical).

The speechnetworkprovidesthetwo to four wires inter­face,electronicswitchingbetweendialingandspeechand
automaticgaincontrolontransmi tandr eceive.

Thediali ngnetwor kbuffersupto32digitsintotheLND
memorythatcanbelaterredi aledwithasi ngleke yinput.
Users can store all 13 signalling keys and acc es s
seve r aluniquefunct ionswit hsingl ekeyentries.T hes e
functionsinclude:Pause,LastNum be rDial led (LN D) ,
Softs w i tch andF l as h.( s eeFigure1).

TheFLASHkeys i mulatesahookflashto transfercalls
or to activateother specialfeaturesprovidedby the
P ABXor centraloffice.

ThePAUSE keystoresa timed pauseinthenumber
sequence. Redialis then delayeduntil an outs i deline
canbeaccessedorsom eotheractivitie soccurbefore
normal signalling resumes. A LND key autom atically
redialsthela st number dial ed.

Adedic atedpinMU/MFI isus edtoselectthemutingfor
transmit ampl i f ier and lighted dial LED for keyboar d
illuminati onoraLEDindicato rforto nemodeindic ation.

TheSEL pin allows selection of any one of the four
poss i ble Flashdurationopti ons.

DIP28

(Plastic Package)

ORDER CODES :

ST3917AN - ST3917BN

ST3917AD - ST3917BD

SO28

(Plastic Package)

ORDER CODES :

July 1996

1/16

ST3917A - ST3917B

PIN CONNECTIONS

C1

C2
C3

SEL

OSC

PULSE

MODE/PT

HKS

GND

RXOUT

GRX

RXIN

I

REF

V

1
2
3
4
5
6
7
8
9
10
11
12
13
14

CC

28
27
26
25
24
23
22
21
20
19
18
17
16
15

R1
R2
R3
R4
MU/MFI
V

DD

GDTMF
MIC+

MIC-

GTX

REG
LN

ILINE

LED

PIN DESCRIPTION
Figure 1 : KeyboardConfiguration Scanning consists of Rows and Columns alter-

natelyswitching highthroughonchippullups.After

123
456

Flash

Tone/Pulse

botha Row andColumnkeyshavebeen detected,
the debounce counter is enabled and any noise
(bouncingcontacts,etc)is ignored for a debounce
period(TKD) of 32ms. At thistime,the keyboardis

789

Pause

sampledandifboth the Row and Column informa­tion are valid, the information is buffered into the

0#

*

C1, C2, C3, R4, R3, R2, R1

(Keyboardinputs,Pins 1, 2, 3, 25,26, 27, 28)

The deviceinterfaces with eitherthe standard 2 of
8 with negative common or the single contact
(Form A) keyboard. Column 4 of the keypad is
connectingto ground.

LND

LNDlocation.

3917-02.EPS

In the tone mode,if two or more keysin thesame
rowor if two or more keys inthe samecolumnare
depressed a single tone will be output. The tone
will correspond to the common row or common
column for which the two keys were pushed.This
featureisfortestpurposes,and singletonewillnot
be redialed.Also in the tonemode,theoutputtone
is continuous in the manual dialing as long as the
keyispushed. Theoutputtonedurationfollowsthe

A valid keypad entry is either a single Row con­nected to a single Columnor GNDsimultaneously
presented to both a single Row and a single Col­umn. In its quiescent or standby state,during nor­mal off-hookoperation,the Rows are initializedat
logic level 1 (V

) and the columns are initialized

DD

at logic level 0 (GND). Pulling any row input low
enables the on chip oscillator.Keyboardscanning
then begins.

Table 1. When redialing in the tone mode, each
DTMF outputis90ms duration,and thetonesepa­ration(inter signal delay) is90ms.

Table 1 : Output ToneDuration

Key Push Time, T Tone Output

T < 32ms

32ms < T < 90ms + Tkd

T > 90 ms + Tkd

Nooutput,ignoredbythe device
90ms duration
Output duration = T - tkd

3917-01.EPS

2/16

PIN DESCRIPTION (continued)

SEL (Input, Pin 4)

This is an option selectable pin for four Flash
duration. The four options are summarised in the
table 2.

For option 1, softswitch feature is inhibited. It
means redialed by the LND key in pulse mode will
not repeat the softswitch and subsequent digits,
only pulse digits are dialed out.

Table 2 : Options Selectable for Flash Duration

Options SEL Flash (ms) Softswitch

1V
2 GND 600 Enable
3 Any Row 300 Enable
4 Any Col 100 Enable

DD

100 Inhibited

OSC (Input, Pin 5)

Only one pin is needed to connect the ceramic
resonatorto the oscillatorcircuit. Theother end of
the resonator is connected to GND. The nominal
resonatorfrequencyis 3.579545MHzand any de­viationfromthisstandardis directly reflectedin the
Tone output frequencies. The ceramic resonator
providesthe timereferencefor all circuitfunctions.
A ceramic resonator with tolerance of ±0.25% is
recommended.

PULSE(Output,Pin 6)

This is an output consisting of an open drain N­channel device. During on-hook, pulse output pin
is in high impedance and once off-hook, it will be
pulledhigh by externalresistor.

MODE/PT(Input, Pin 7)

Input (MODE). MODE determinesthe dialer’s de­fault operatingmode.Whenthedevice is powered
up or the hookswitch input is switched from on­hook (V
mines the signalling mode. A V

) to off-hook (GND), the default deter-

DD

connection

DD

defaults to tone mode operation and a GND con­nectiondefaults to pulse mode operation.

When dialing in the pulse mode, a softswitch fea­turewillallowa changetothe tonemodewhenever
the * or softswitchkey (TONE) is depressed. Sub­sequent * key inputs willcausethe DTMF codefor
an * to be dialed. The softswitch will only switch
frompulseto tone.Thephonewillbe inpulsemode
only after returning to on-hook and back to off­hook. Redialed by the LND key will repeat the
softswitch unless the softswitch redial feature is
inhibited.

Output(PT). Pacifier Tone Output. In pulse mode,

ST3917A - ST3917B

all valid key entries activate the pacifier tone. In
tonemode,anynonDTMF entry (FLASH,PAUSE,
LND, TONE) activates the pacifier tone. The paci­fiertoneprovidesaudiblefeedback,confirmingthat
the key has been properly entered and accepted.
It is a 500Hz square wave activated upon accep­tance of valid key input after the 32ms debounce
time.

Thesquarewaveterminatesafter75mstypicallyor
whenthevalidkeyisnolongerpresent.Thepacifier
tonesignal is simultaneouslysentto the earphone
and the buzzer.The buzzer can be removedwith­outaffectingthis function.Theresistorvalueseton
MODE/PTpin determines the level of the pacifier
tone in theearphone.

HKS (Input, Pin8)

This is the hookswitch input to the device. It is a
CMOSinput with a high pull up internal resistance
and must be switched high or open for on-hook
operationand low for off-hook operation. A transi­tionon this input causestheon-chiplogictoinitial­ize, terminating any operation in progress at the
time. The signalling mode defaults to the mode
selected at MODE/PT pin. Figures 2, 3 and 4, 5
illustratethe timing for this pin.

GND (Pin 9)

GNDisthenegativelineterminalofthedevice.This
isthe voltagereferencefor all specifications.

RXOUT, GRX, RXIN (Pins 10, 11and 12)

ThereceiveamplifierhasoneinputRXINandanon
inverting output RXOUT. Amplification from RXIN
to RXOUTis typically31dBand it can be adjusted
between 21dB and 41dB to suit the sensitivity of
the earphone used. The amplification is propor­tional to the external resistor connected between
GRX and RXOUT. For the hearing impaired, a
specificapplicationto offer17dBadditionalgainat
3kHzis permitted.

I

(Pin 13)

REF

An external resistor of 3.6kΩ connectedbetween
IREF and GND will set the internal current level.
Anychangeof this resistor value will influence the
microphone gain, DTMF gain, earphone gain and
sidetonelevel.

VCC(Pin14)

VCCis the positivesupplyof the speechnetwork. It
can be stabilized by a decoupling capacitor be­tween V
alsobe usedtosupplyexternalperipheralcircuits.

andGND. The VCCsupplyvoltagemay

CC

3/16

ST3917A - ST3917B

PIN DESCRIPTION (continued)

LED (Output,Pin 15)

When the MU/MFI pin is connected to either V
or GND,theLED connectedto the LED pin,which
functions as a keyboard illumination or off-hook
indicator, will light up when the telephone is off­hook.
WhentheMU/MFIpinisconnectedto anyrow pins,
the LED connectedto LED pin functionsasa tone
mode indicator.
Fromminimum operating line current up to 20mA,
I

LN-ICC

is sourced into the LED with a maximum
current limit of 18mA. For line current more than
20mA, this sourced current is limited at 18mA
(typical).

DD

GDTMF(Pin22)

Whenthe DTMF input is enabled,the microphone
inputsandthereceiveamplifier input will be muted
and the dialing tone will be sent on the line. The
voltage amplification from GDTMF to LN is 40dB.
Finaloutputlevelon the LN canbeadjustedviathe
external resistor connectedbetween GDTMF and
GNDthrougha decouplingcapacitor.Aconfidence
tone is sent to the earphone during tone dialing.
Theattenuation of the confidence tone from LN to
RXOUT is -32dB typically. The level of the confi­dence tone in the earphone can be increased by
adjustingthe resistor connected between GDTMF
and GRX pins, the possiblerange is 20dB.

ILINE (Pin 16)

A recommended external resistor of 20Ω is con­nected between ILINE and GND. Changing this
resistor value will influence the microphone gain,
DTMF gain, sidetone, maximum output swing on
LN andtheDCcharacteristics,especiallyinthe low
voltageregion.

LN (Pin 17)

LN is the positive line terminal of the device.

REG (Pin 18)

The internalvoltageregulatorhas to bedecoupled
by a capacitorfrom REGto GND.TheDC charac­teristics can be changed with an external resistor
connectedbetweenLN and REGor betweenREG
and ILINE.

GTX,MIC-, MIC+ (Pins 19, 20, 21)

The devicehas a symmetricalmicrophoneinputs.
The amplification from microphoneinputs to LN is
51dB at 15mAline current and it can be adjusted
between43 and51dB.Theamplificationispropor­tional to the external resistor connected between
GTX and REG.

VDD(Pin23)

VDDis the positive supplyforthe dialingcircuit and
it must meet the maximum and minimum voltage
requirements.

MU/MFI (Input, Pin 24)

Alogic lowinput to this pin willdisablethetransmit
amplifier of the speech circuit. MUTE efficiency is
greater than 60dB. An open circuit to this pin will
enable the transmit amplifier. In this case, LED is
usedfor keypadlighting.

A connection to any row will disable the transmit
mute function and the LED connectedto the LED
pin is used for tonemode indication.

Table 3 : Logicof MU/MFI Pin Indicator

MU/MFI

Pin

OPEN Active Lighted Dial Indicator

GND Muted LightedDial Indicator

Any Row Not Available Tone Mode Indicator

Transmit

Muting

LED at Pin 15

4/16

BLOCK DIAGRAM

ST3917A - ST3917B

LA

LB

Flash

Tone/Pulse

123

456

Ri

MODEHKSSELPULSELNLED

4678

RAM

REDIAL

PULSE

INTERFACE

LND

Pause

0#

*

789

Ci

24

KEYBOARD

INTERFACE

MUTE

Secret

OSC

MU/MFI

5

OSCILLATOR

DTMF

GENERATOR

LOGIC + DAC

Ceramic

3.58MHz

Ri1

GRX

11

gm

RGRX+C9

RXOUT

10

REF

KI

Ri2

gm

REF

KI

I1

9

12

LN

R24 R25

C20 C11

R15

15 1723

CONTROL

ILINE/ILED

R21

R20RGTX

16 1819

V1

DD

V

GTX ILINE REG RXIN GND

+

C8

D12

IREF

REF

KI

gm

gm

kI

REF

ST3917A/B

13

REF

I

R14

22

GDTMF

RMFCMF

MIC+

20

21

MIC-

C14

3917-03.EPS

5/16

V

C10

14

CC