Preliminary Data Sheet

AS2507

3/9

July 1998

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.

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 600

Ω 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

600

Ω).

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

impedance is 60k

Ω.

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

Ω resistor.

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

TXD

MODE

0

0

IDLE, no transmission

0

1

TEST, for testing only

1

0

SPACE ("0") 18.645 kHz

1

1

MARK ("1") 22.375 kHz

FSK Demodulator

Also the demodulator provides two modes for

detection as follows:

Signal on line

RXD

IDLE, no transmission

1

SPACE ("0") 18.645 kHz

0

MARK ("1") 22.375 kHz

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

a

header

(e.g.

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.