Data Sheet AS2540

austria

micro

systems

Revision 3.1

Page 1 of 18

austria

micro

systems

General Description

The AS2540 is a CMOS integrated circuit that
incorporates a speech circuit (line-adaptation, 2/4-wire
conversion, separate Rx- and Tx-amplifiers for handset
and modem), MF dialler, ring frequency detector, ring
melody generator and a two wire serial interface. It
shall act as an a/b-line-powered device in order to
interface a main-powered CPU to the analogue
telephone line.

AS2540 can operate in a so-called mainpower mode
and in a backup mode. During mainpower mode, a
watchdog timer is running on AS2540, which has to be
reset via the serial interface, which is connected to the
CPU through opto-couplers. During backup mode (=
The main-power failure or SW engine problem),
AS2540 can operate as an independent basic MF
dialler (digits 0-9*#)

Applications

Web Telephones

Answering Machines

Fax Machines

Base Station of Cordless Telephones

Hospital Communication Units

Key Features

Line Interface, Speech Circuit, MF Dialler and Tone Ringer
on a 28 pin CMOS chip

Programmable via 2-wire serial interface
(UART 9600 baud, 8N1)

Watch dog timer for Backup Mode

Additional input for modem transmit path

Additional output for modem receive path

CMOS for high EMC immunity

Low Noise (max. �72 dBm)

Operating range from 15mA to 100mA (down to 5mA with
reduced performance)

Soft clipping for handset operation to avoid harsh distortion

Line loss compensation selectable by pin option

Real and complex impedance selectable by external
components

Side tone adaption selectable by external components

Ring frequency discrimination

3-tone ring melody generator

Package

28 pin SOIC

Dice-on-Foil

Block Diagram

ROH

ROT

M1
M2

TIT

AGND

Line

STB

DPn

C1
C2
C3

R1
R2

MODE

RFD

-2dB

+ 6dB

+ 2dB

Mask

DTMF-

generator

DIALLER

UART

I
T

U
T
E

Dia

Dig

i
t

S
K

P
n

RxD

TxD

VDD

EHS

MHS

VSS

OSC

Main Power Mode

ROH

ROT

M1
M2

TIT

AGND

Line

STB

DPn

C1
C2
C3

R1
R2

MODE

RFD

-2dB

+ 6dB

+ 2dB

Mask

DTMF-

generator

DIALLER

UART

I
T

U
T
E

Dia

Dig

i
t

S
MA

S
K

S
DP

RxD

TxD

VDD

EHS

MHS

VSS

OSC

Backup Mode

Remote Controlled Single-Chip-Telephone

AS2540

DATA SHEET (PRELIMINARY)

Data Sheet AS2540

austria

micro

systems

Revision 3.1

Page 2 of 18

Pin Description

Pin #

Symbol

Function

RFD

ing

requency

iscrimination

Schmitt trigger input for ring frequency discrimination. Disabled during off-hook.

ine Current

ense Input

Input for sensing the line current

ine

nput

Input used for power extraction and line current sensing

eceive

nput

Input for ac-separated receive signal

STB

ide

one

alance Input

Input for side tone cancellation network

urrent

hunt Control Output

N-channel open drain output to control the external high power shunt transistor for synthesising ac- and
dc- impedance, modulation of line voltage and shorting the line during make periods of pulse dialling

omplex

mpedance Input

Input pin for the capacitor to program a complex impedance

DPn

ial

ulse Output

Open drain output pulls low during break periods of pulse dialling and flash

EHS

lectronic

ook

witch

Schmitt Trigger Input to detect an electronic off-hook.

MHS

anual

ook

witch

Schmitt Trigger Input to detect a manual off-hook.

MODE

TMF

evel

election

-4/-6dBm

--> DLS open

-6/-8dBm

--> DLS-r1

-8/-10dBm

--> DLS-r2

ine

oss

ompensation Select Pin

45mA-75mA

--> LLC open

20mA-50mA

--> LLC-r3

None

--> LLC-r4

Ring

elody

utput

PDM output of the ring melody generator.

VDD

oltage

rain

Positive Power Supply

VSS

oltage

ource

Negative Power Supply

Data Sheet AS2540

austria

micro

systems

Revision 3.1

Page 3 of 18

Pin #

Symbol

Function

OSC

scillator Input

Input for ceramic resonator 3.58MHz.
Recommended type: Murata CSA 3.58MG312AM or compatible

AGND

nalogue

round

Special ground for the internal amplifiers

RXD

eceive

ata Input

Schmitt trigger input to serial interface

TXD

ransmit

ata Output

Open drain output from serial interface

20
21

R1
R2

Keyboard

ows

22
23
24

C1
C2
C3

Keyboard

olumns

ROT

eceive

utput to

ransformer

Output for driving a transformer with an ac impedance larger than 10k

TIT

ransformer

nput from

ransformer

Single ended input for transformer

ROH

eceive

utput to

andset

Output for driving a dynamic earpiece with an impedance from 150

to 300

13
14

M2
M1

icrophone Inputs

Differential inputs for the microphone (electret)

Data Sheet AS2540

austria

micro

systems

Revision 3.1

Page 4 of 18

Functional Description

Modes of operation

There are 4 possible modes:

Idle Mode

Manual hook-switch and electronic hook-switch (e.g. relay)
is open (MHS

EHS = logical 0), no ring signal is applied,

the oscillator is stopped, signalling via the serial interface
is not possible.

Ringing Mode

AS2540 is supplied via the ring capacitor and external
path, manual hook -switch and electronic hook-switch is
open (MHS

EHS = logical 0), the oscillator is running, a

valid incoming ringing is signalled via the serial interface to
the CPU. If the CPU does not reply the AS2540 switches
into backup mode and sends a ring melody to the MO
output.

Backup Mode

Manual hook-switch and/or electronic hook-switch is closed
(MHS

EHS = logical 1), AS2540 is supplied with line

current, the oscillator is running, the keyscan is enabled
and the handset path is switched on, signalling via the
serial interface is possible. After AS2540 has started
dialling in backup mode, he must not be interrupted any
more in order to ensure consistent dialling. A mode
transition BM -> MPM is only possible before AS2540 has
started dialling digits independently.

Main Power Mode

Manual hook-switch and/or electronic hook-switch is
closed, AS2540 is supplied with line current (MHS

EHS

= logical 1), the oscillator is running, the keyscan is
disabled, AS2540 is controlled via the serial interface,
handset path or monitor path or modem path is on,
signalling via the serial interface is possible.

Select Operation Mode

Command Message

Switches ON

Switches OFF

SROH

SROT & STIT

SROT & STIT

SROH & SROT

STIT

Example Main Power Mode

ROH

ROT

M1
M2

TIT

AGND

Line

STB

DPn

C1
C2
C3

R1
R2

MODE

RFD

-2dB

+ 6dB

+ 2dB

Mask

DTMF-

generator

DIALLER

UART

T
E

i
t

S
K

P
n

RxD

TxD

VDD

EHS

MHS

VSS

OSC

Start-up in backup mode

As soon as AS2540 is supplied with line current, the
external V

capacitor will be charged up via the LI and

VDD pin and after the V

voltage has reached the

operating level, the handset path of the speech circuit is
switched on. This off-hook status is also signalled to the
internal logic via the MHS or EHS pin in order to start the
pinscan and subsequently set the country specific modes.
Additionally a <LineEvent> message is sent to the CPU.

Start-up in ring mode

When a ringing signal is applied to the line, V

of AS2540

is charged up via an external path. After V

has reached

the operating voltage the oscillator starts and AS2540
discriminates the ring frequency. After a valid ring
frequency is applied on the RFD pin, the logic sends a
<LineEvent> message is sent to the CPU and a watchdog
timer is started. When the <LineEvent> message is not quit
by the CPU before the watchdog timer has finished the
countdown, AS2540 changes into backup mode and sends
out a melody via the MO pin.

DC conditions

The normal operating mode is from 15mA to 100mA. An
operating mode with reduced performance is from 5mA to
15mA. In the line hold range from 0mA to 5mA the device
is in a power down mode and the voltage at LI is reduced
to a maximum of 3.5V

Data Sheet AS2540

austria

micro

systems

Revision 3.1

Page 5 of 18

The DC characteristic is determined by the voltage at LI
pin and a 30

resistor between LI and LS pin. It can be

calculated by the following equation: V

= V

+ I

Line

. The t.m. voltage at the LI pin is 4.5V. The calculation

leads to the following DC resistances: 330

to 75

at line

currents from 15mA to 100mA.

2/4 wire conversion

AS2540 has a built-in dual Wheatstone bridge with one
common ground. This provides a maximum of
independence of AC impedance and side tone from each
other. One can adapt side tone without changing the AC
impedance.

A1=3.4/3.5

A2=-0.4

AC impedance

The ac impedance of AS2540 is set to t.m. 1000

. With

the external capacitor at CI pin it can be programmed
complex. With a parallel impedance at the LS pin it can be
programmed to 600

Side Tone

A good sidetone cancellation can be achieved by using the
following equation:

BAL

LINE

= 10

Transmit path

The gain of the M1/M2 --> LS transmit path is set to 38dB
for ac-impedance 1000

. The input is differential with an

impedance of 25k

. The soft clip circuit limits the output

voltage at LS to 2V

The attack time is 30�s/6dB and the

decay time is 20ms/6dB.

The gain of the TIT --> LS transmit path is set to 8dB for
ac-impedance 1000

. The input is single ended with an

impedance of 25k

. The soft clip circuit is not active for

this input. The dynamic range of this path is 2.5Vp at LS.

Receive path

The gain of the LS --> ROH receive path is set to 2dB for
ac-impedance 1000

. The receive input is the differential

signal of RI and STB. During DTMF dialling a MF comfort
tone is applied to ROH which is -30dB relative to the line
level. The soft clip circuit limits the output voltage at ROH
to 1V

It prevents harsh distortion and acoustic shock.

The gain of the LS --> ROT receive path is set to 0dB for
ac-impedance 1000

. The receive input is the differential

signal of RI and STB. The soft clip circuit is not active for
this output.

Modescan

The DTMF-level on the line and the function of the line loss
compensation can be selected with the connection of the
MODE-pin to the keyboard matrix. The MODE-pin is
scanned once after each OFF-Hook transition.

PIN

MODE

LLC

Dtmf Level

45mA-75mA

-6dBm

45mA-75mA

-8dBm

45mA-75mA

-10dBm

20mA-50mA

-6dBm

20mA-50mA

-8dBm

20mA-50mA

-10dBm

No LLC

-6dBm

No LLC

-8dBm

No LLC

-10dBm

X indicates connection to pin mode

Электронный компонент: AS2540Q