Preliminary Data Sheet
AS2507
®
Austria Mikro Systeme International AG
Single Chip 2-Wire Intercom
CMOS Integrated Circuit
General Description
The AS2507 is a CMOS integrated circuit that
contains all the functions needed to build a 2-
wire intercom network.
The device incorporates 2/4-wire conversion
(hybrid), soft clipping for high speech quality,
FSK modem and a simple interface to a
microcontroller.
The signalling mode is selectable between
FSK modulation and burst mode.
The low standby current (typ. 2 mA) allows
several devices to listen to the 2-wire line.
The speech circuit is designed for compatibility
with commonly used handset (150Ω earpiece
and electret microphone) with receive gain of
-6 dB and transmit gain of 32 dB (relative to
line).
Key Features
Line/speech circuit and signalling on one 14
pin CMOS chip
Only 2 wires needed for power supply,
signalling and speech
Soft clipping to avoid harsh distortion
Fully integrated 2/4 wire conversion
Side tone cancellation
Low noise
Signalling with FSK modem
Low standby power consumption allows
parallel operation of up to 25 terminals on a
bus pair with central supply
Parallel operation of up to 70 terminals if
supplied locally
Controllable via simple µC interface
Very few external components
Application
Entrance telephone system, intercom and data
transmission, alarm systems, toy phone
Package
Available in 14 pin DIP and 16 pin SOIC
Block Diagram
1/9
July 1998
Preliminary Data Sheet
AS2507
Pin description
Pin #
DIP 14
7
6
2
4
Pin #
SO 16
7
6
2
4
Symbol
Function
M1
M2
RI
RO
M
icrophone Input
1
Differential input for the microphone (electret)
M
icrophone Input
2
Differential input for the microphone (electret)
R
eceive
I
nput
Input for ac-separated receive signal
R
eceive
O
utput to Handset
Output for driving a dynamic earpiece with an impedance from 150Ω to
300Ω
14
9
8
11
12
5
15
16
9
10
8
12
13
5
VDD
VSS
AGND
OSC1
OSC2
PD
V
oltage
D
rain
D
rain
Positive Power Supply
V
oltage
S
ource
S
ource
Negative Power Supply
A
nalogue
G
round
Signal ground for the internal amplifiers
O
scillator Output
1
Output to ceramic resonator 3.58MHz.
O
scillator Input
2
Input for ceramic resonator 3.58MHz.
P
ower
D
own Input
Active high, i.e. a high level on this pin will power down the analogue
signal path.
10
13
1
11
14
1
RxD
TxD
TE
R
eceive
D
ata Output
Output of the FSK demodulator
T
ransmit
D
ata Input
Input for the FSK modulator
T
ransmit
E
nable
Input for enabling transmit data
2/9
July 1998
Preliminary Data Sheet
AS2507
Functional Description
The AS2507 is a CMOS integrated circuit that
incorporates a speech circuit and a FSK
modem. It is intended to be used as line-
powered interface on a 2-wire intercom bus.
FSK Modulator
Two signalling modes are provided, namely
FSK and burst mode. The signalling mode and
the signalling can be controlled by a
microcontroller using the TE and TXD inputs
as follows:
TE
0
0
1
1
TXD
0
1
0
1
MODE
IDLE, no transmission
TEST, for testing only
SPACE ("0") 18.645 kHz
MARK ("1") 22.375 kHz
Standby Condition
During standby operation (PD=High) only the
FSK demodulator is active to provide the
companion microcontroller with all signalling
information on the 2-wire bus. The low power
consumption (typical 2mA) and a high input
and output impedance in standby mode allow
the parallel operation of many terminals on
each bus pair.
2/4-Wire Conversion
The AS2507 has a built-in side tone
cancellation circuit. The transmit signal is
attenuated by 6 dB over the 600Ω resistor (ac
impedance) and subtracted from the receive
signal at the receive input (RI). This
configuration allows the selection of the
required ac impedance and yet maintaining a
good side tone cancellation.
AC Impedance
The ac impedance is determined by an
external resistor at the TO output (typical
600Ω).
Transmit Path
The gain of the transmit path from the
microphone inputs (M1/M2) to the transmit
output (TO) is set to 32 dB. The soft clip level
is set to 0.5V
PEAK
at TO. In standby the output
impedance is 60kΩ.
Receive Path
The gain of the receive path from the receive
input (RI) to the receive output (RO) is set to
-6dB. The receive input is internally biased to
A
GND
with a 500kΩ resistor.
FSK Demodulator
Also the demodulator provides two modes for
detection as follows:
Signal on line
IDLE, no transmission
SPACE ("0") 18.645 kHz
MARK ("1") 22.375 kHz
RXD
1
0
1
The demodulator consists out of a bandpass
filter to attenuate interfering speech signals
and a period counter. The RxD output is
updated after each valid period or a counter
overflow.
Transmission Protocol
In order to assure a safe data transmission,
data framing is recommended. Each frame
should consist out of a preamble (e.g. FF
hex
),
a header (e.g. AA
hex
), data bytes and
checksum.
Independence of bus polarity
Centrally supplied terminals can be connected
to the 2-wire bus independent from polarity. In
this case a rectifier bridge, known from
telephony, is recommended since the DC-
supply current will bias the diodes.
3/9
July 1998
Preliminary Data Sheet
AS2507
Typical 2-Wire Intercom Application
4/9
July 1998
Preliminary Data Sheet
AS2507
Electrical characteristics
Electrical charateristics are measured with the Test Circuit application. Typical mean values will not be
tested.
Absolute maximum ratings
Positive Supply Voltage
Input Current
Analogue Input Voltage
Digital Input Voltage
Electrostatic Discharge (HBM 1.5kΩ-100pF)
Storage Temperature
-0.3V <= V
DD
<= 7V
± 25mA
-0.3V <= V
in
<= V
DD
+0.3V
-0.3V <= V
in
<= V
DD
+0.3V
± 1000V
-65°C to +125°C
Recommended operating conditions
Supply Voltage (V
DD
)
Oscillator Frequency
Operating Temperature
5V ±10%
3.58 MHz
-10°C to +60°C
DC characteristics
V
DD
=5V, f=1kHz, unless other specified
Symbol Parameter
I
DDS
I
DDO
V
IL
V
IH
Supply Current
Supply Current
Digital Input Voltage LOW
Digital Input Voltage HIGH
Conditions
Standby Mode
PD=High, V
DD
=2.5V
Operating Mode
PD=Low, V
DD
=2.5V
V
SS
0.9 V
DD
Min
Typ
2
5
Max
3
6
0.1 V
DD
V
DD
Units
mA
mA
V
V
5/9
July 1998
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