ST3917A
ST3917B
SPEECH - TONE/PULSE DIALER - LED INDICATOR
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SPEECH CIRCUIT
2 TO 4 WIRES CONVERSION
PRESENT THE PROPER DC PATH FOR THE LINE
CURRENT AND THE FLEXIBILITY TO ADJUST IT
ANDALLOWPARALLEL PHONE OPERATION
SYMMETRICAL HIGH IMPEDANCE MICRO-
PHONE INPUTS SUITABLE FOR DYNAMIC,
ELECTRET OR PIEZOELECTRIC TRANSDUCER
ASYMMETRICAL EARPHONE OUTPUT SUIT-
ABLE FOR DYNAMIC TRANSDUCER
LINE LOSS COMPENSATION FIXED INTER-
NALLY BY A.G.C.
INTERNAL MUTING TO DISABLE SPEECH
DURING DIALING AND EXTERNAL MUTING
TO DISABLE TRANSMIT AMPLIFIER DURING
CONVERSATION MODE
LED INDICATOR EITHER FOR KEYBOARD ILLU-
MINATION (OR ON-LINE INDICATION) DURING
DIALING AND CONVERSATION OR FOR TONE
MODE INDICATION, THROUGH MU/MFI PIN
DIALER CIRCUIT
STORE UP TO 32 DIGITS FOR LAST NUMBER
REDIAL BUFFER, LNR IS INHIBITED IF THERE
ARE MORE THAN 32 DIGITS STORED
ALLOWMIXED MODE DIALING IN PULSE MODE
PACIFIER TONE PROVIDES AUDIBLE INDI-
CATION OF VALID KEY PRESSED IN A BUZZ-
ER OR/AND IN THE EARPHONE
TIMED PABX PAUSE / 10PPS PULSE RATE
MAKE/BREAK RATIO : 40/60 (ST3917A) AND
33/67 (ST3917B)
4 SELECTABLE OPTIONS ON FLASH DURA-
TION AND SOFTSWITCH INHIBITION IN ONE
OF THE OPTION WITH 100ms FLASH TIMING
2 SELECTABLE OPTIONS : TRANSMIT MUTE
TOGETHER WITH LED FOR KEYBOARD ILLUMI-
NATION OR LED FOR TONE MODE INDICATION
CONTINUOUS TONE FOR EACH DIGIT UNTIL
KEY RELEASE
USES INEXPENSIVE 3.579545MHz CERAMIC
RESONATOR
POWERED FROM TELEPHONE LINE, LOW
OPERATING VOLTAGE FOR LONG LOOP AP-
PLICATION
July 1996
DESCRIPTION
The device consists of the speech and the dialer func-
tions. It provides the DC line interface circuit that termi-
nates the telephone line, analog amplifier for speech
transmission and necessary signals for either DTMF
and pulse dialing. When mated with a tone ringer, a
complete telephonecan be producedwith just two ICs.
TheDClineinterfacecircuit developsits ownlinevoltage
acrossthedeviceandit is adjustableby externalresistor
to suit different country’s specification. A built-in LED
driver can deliver excess line current to external LED
indicator(s) during dialing and speech mode. The
LED(s) can be used either for keyboard illumination
purpose or for tone mode indication during softswitch
and mixed mode dialing by connecting MU/MFI pin to
VDDandGND oranyrowrespectively.The LEDcurrent
is limited to 17mA (typical).
The speech network provides the two to four wires inter-
face,electronic switchingbetweendialingandspeechand
automaticgain control on transmit and receive.
The dialing network buffers up to 32 digits into the LND
memorythatcan be laterredialedwith asinglekeyinput.
Users can store all 13 signalling keys and access
several unique functions with single key entries. These
functions include : Pause, Last Number Dialled (LND),
Softswitch and Flash. (see Figure 1).
The FLASH key simulates a hookflash to transfercalls
or to activate other special features provided by the
PABX or central office.
The PAUSE key stores a timed pause in the number
sequence. Redial is then delayed until an outside line
can be accessed or some other activities occur before
normal signalling resumes. A LND key automatically
redials the last number dialed.
Adedicatedpin MU/MFI is used to select the muting for
transmit amplifier and lighted dial LED for keyboard
illumination or a LEDindicator for tonemode indication.
The SEL pin allows selection of any one of the four
possible Flash duration options.
DIP28
(Plastic Package)
ORDER CODES :
ST3917AN - ST3917BN
SO28
(Plastic Package)
ORDER CODES :
ST3917AD - ST3917BD
1/16
ST3917A - ST3917B
PIN CONNECTIONS
C1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
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-
C2
C3
SEL
OSC
PULSE
MODE/PT
HKS
GND
RXOUT
GRX
RXIN
I
REF
V
CC
GTX
REG
LN
3917-01.EPS
ILINE
LED
PIN DESCRIPTION
Figure 1 :
Keyboard Configuration
1
4
7
*
2
5
8
0
3
6
9
#
Flas h
Tone /P ulse
P a us e
LND
3917-02.EPS
Scanning consists of Rows and Columns alter-
nately switching high through on chip pullups. After
both a Row and Column keys have been detected,
the debounce counter is enabled and any noise
(bouncing contacts, etc) is ignored for a debounce
period (TKD) of 32ms. At this time, the keyboard is
sampled and if both the Row and Column informa-
tion are valid, the information is buffered into the
LND location.
In the tone mode, if two or more keys in the same
row or if two or more keys in the same column are
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
feature is for test purposes, and single tone will not
be redialed. Also in the tone mode, the output tone
is continuous in the manual dialing as long as the
key is pushed. The output tone duration follows the
Table 1. When redialing in the tone mode, each
DTMF output is 90ms duration, and the tone sepa-
ration (inter signal delay) is 90ms.
Table 1 :
Output Tone Duration
Key Push Time, T
Tone Output
T < 32ms
No output, ignored by the device
32ms < T < 90ms + Tkd 90ms duration
T > 90 ms + Tkd
Output duration = T - tkd
C1, C2, C3, R4, R3, R2, R1
(Keyboard inputs, Pins 1, 2, 3, 25, 26, 27, 28)
The device interfaces with either the standard 2 of
8 with negative common or the single contact
(Form A) keyboard. Column 4 of the keypad is
connecting to ground.
A valid keypad entry is either a single Row con-
nected to a single Column or GND simultaneously
presented to both a single Row and a single Col-
umn. In its quiescent or standby state, during nor-
mal off-hook operation, the Rows are initialized at
logic level 1 (V
DD
) and the columns are initialized
at logic level 0 (GND). Pulling any row input low
enables the on chip oscillator. Keyboard scanning
then begins.
2/16
ST3917A - ST3917B
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
1
2
3
4
SEL
V
DD
GND
Any Row
Any Col
Flash (ms)
100
600
300
100
Softswitch
Inhibited
Enable
Enable
Enable
all valid key entries activate the pacifier tone. In
tone mode, any non DTMF entry (FLASH, PAUSE,
LND, TONE) activates the pacifier tone. The paci-
fier tone provides audible feedback, confirmingthat
the key has been properly entered and accepted.
It is a 500Hz square wave activated upon accep-
tance of valid key input after the 32ms debounce
time.
The square wave terminates after 75ms typically or
when the valid key is no longer present. The pacifier
tone signal is simultaneously sent to the earphone
and the buzzer. The buzzer can be removed with-
out affecting this function. The resistor value set on
MODE/PT pin determines the level of the pacifier
tone in the earphone.
OSC
(Input, Pin 5)
Only one pin is needed to connect the ceramic
resonator to the oscillator circuit. The other end of
the resonator is connected to GND. The nominal
resonator frequency is 3.579545MHz and any de-
viation from this standard is directly reflected in the
Tone output frequencies. The ceramic resonator
provides the time reference for all circuit functions.
A ceramic resonator with tolerance of
±0.25%
is
recommended.
PULSE
(Output, Pin 6)
This is an output consisting of an open drain N-
channel device. During on-hook, pulse output pin
is in high impedance and once off-hook, it will be
pulled high by external resistor.
MODE/PT
(Input, Pin 7)
Input (MODE). MODE determines the dialer’s de-
fault operating mode. When the device is powered
up or the hookswitch input is switched from on-
hook (V
DD
) to off-hook (GND), the default deter-
mines the signalling mode. A V
DD
connection
defaults to tone mode operation and a GND con-
nection defaults to pulse mode operation.
When dialing in the pulse mode, a softswitch fea-
ture will allow a change to the tone mode whenever
the * or softswitch key (TONE) is depressed. Sub-
sequent * key inputs will cause the DTMF code for
an * to be dialed. The softswitch will only switch
from pulse to tone. The phone will be in pulse mode
only after returning to on-hook and back to off-
hook. Redialed by the LND key will repeat the
softswitch unless the softswitch redial feature is
inhibited.
Output (PT). Pacifier Tone Output. In pulse mode,
HKS
(Input, Pin 8)
This is the hookswitch input to the device. It is a
CMOS input with a high pull up internal resistance
and must be switched high or open for on-hook
operation and low for off-hook operation. A transi-
tion on this input causes the on-chip logic to initial-
ize, 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
illustrate the timing for this pin.
GND
(Pin 9)
GND is the negative line terminal of the device.This
is the voltage reference for all specifications.
RXOUT, GRX, RXIN
(Pins 10, 11 and 12)
The receive amplifier has one input RXIN and a non
inverting output RXOUT. Amplification from RXIN
to RXOUT is typically 31dB and it can be adjusted
between 21dB and 41dB to suit the sensitivity of
the earphone used. The amplification is propor-
tional to the external resistor connected between
GRX and RXOUT. For the hearing impaired, a
specific application to offer 17dB additional gain at
3kHz is permitted.
I
REF
(Pin 13)
An external resistor of 3.6kΩ connected between
IREF and GND will set the internal current level.
Any change of this resistor value will influence the
microphone gain, DTMF gain, earphone gain and
sidetone level.
V
CC
(Pin 14)
V
CC
is the positive supply of the speech network. It
can be stabilized by a decoupling capacitor be-
tween V
CC
and GND. The V
CC
supply voltage may
also be used to supply external peripheral circuits.
3/16
ST3917A - ST3917B
PIN DESCRIPTION
(continued)
LED
(Output, Pin 15)
When the MU/MFI pin is connected to either V
DD
or GND, the LED connected to the LED pin, which
functions as a keyboard illumination or off-hook
indicator, will light up when the telephone is off-
hook.
When the MU/MFI pin is connected to anyrow pins,
the LED connected to LED pin functions as a tone
mode indicator.
From minimum operating line current up to 20mA,
I
LN
-I
CC
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).
ILINE
(Pin 16)
A recommended external resistor of 20Ω is con-
nected between ILINE and GND. Changing this
resistor value will influence the microphone gain,
DTMF gain, sidetone, maximum output swing on
LN and the DC characteristics, especially in the low
voltage region.
LN
(Pin 17)
LN is the positive line terminal of the device.
REG
(Pin 18)
The internal voltage regulator has to be decoupled
by a capacitor from REG to GND. The DC charac-
teristics can be changed with an external resistor
connected between LN and REG or between REG
and ILINE.
GTX, MIC-, MIC+
(Pins 19, 20, 21)
The device has a symmetrical microphone inputs.
The amplification from microphone inputs to LN is
51dB at 15mA line current and it can be adjusted
between 43 and 51dB. The amplification is propor-
tional to the external resistor connected between
GTX and REG.
GDTMF
(Pin 22)
When the DTMF input is enabled, the microphone
inputs and the receive amplifier input will be muted
and the dialing tone will be sent on the line. The
voltage amplification from GDTMF to LN is 40dB.
Final output level on the LN can be adjusted via the
external resistor connected between GDTMF and
GND through a decoupling capacitor. A confidence
tone is sent to the earphone during tone dialing.
The attenuation of the confidence tone from LN to
RXOUT is -32dB typically. The level of the confi-
dence tone in the earphone can be increased by
adjusting the resistor connected between GDTMF
and GRX pins, the possible range is 20dB.
V
DD
(Pin 23)
V
DD
is the positive supply for the dialing circuit and
it must meet the maximum and minimum voltage
requirements.
MU/MFI
(Input, Pin 24)
A logic low input to this pin will disable the transmit
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
used for keypad lighting.
A connection to any row will disable the transmit
mute function and the LED connected to the LED
pin is used for tone mode indication.
Table 3 :
Logic of MU/MFI Pin Indicator
MU/MFI
Pin
OPEN
GND
Any Row
Transmit
Muting
Active
Muted
Not Available
LED at Pin 15
Lighted Dial Indicator
Lighted Dial Indicator
Tone Mode Indicator
4/16
R15
+
LA
V
DD
LED
15
6
4
8
7
17
D12
LN
LB
1
Ri
7
MUTE
Ci
24
C8
PULSE
SEL HKS MODE
BLOCK DIAGRAM
23
ST3917A/B
PULSE
INTERFACE
4
5
8
*
0
KEYBOARD
INTERFACE
MU/MFI
OSC
RAM
REDIAL
2
3
6
9
#
Flash
Tone/Pulse
Pause
LND
C10
ILINE/ILED
CONTROL
OSCILLATOR
5
V
CC
DTMF
GENERATOR
LOGIC + DAC
14
I
REF
Ri1
Secret
3.58MHz
Ceramic
11
13
IREF
GRX
R14
gm
GDTMF
22
gm
K I
REF
Ri2
gm
V1
I1
K I
REF
RGRX
10
CMF
RMF
K I
REF
MIC+
21
RXOUT
+
C9
C14
gm
MIC-
20
k I
REF
19
16
18
12
9
GTX
RGTX
R21
R20
C20
ILINE
REG
RXIN
C11
GND
LN
R24
R25
ST3917A - ST3917B
5/16
3917-03.EPS
Microcontroller MCU
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