5-3

MT8840

Data Over Voice Modem

Features

Performs ASK (amplitude shift keyed)
modulation and demodulation

32 kHz carrier frequency

Up to 2 kbit/s full duplex data transfer rate

On-chip oscillator

On-chip tone caller for alerting functions

Adjustable tone caller frequencies

Selectable self-loop test mode

5V/2.5mA power supply

ISO

-CMOS and switched capacitor

technologies

18 Pin DIP

Applications

Simultaneous data and voice communication in
PABXs

2 kbit/s data modem

"Smart" telephone sets

Description

The MT8840 is a carrier over voice modem which
allows simultaneous transfer of voice and data over a
single pair of wires. Data is transferred on an
amplitude shift keyed (ASK) 32 kHz carrier. On-chip
filters remove voice frequency signals from the
received composite voice and data signal prior to
demodulation. The modulating signal is a bit stream
with a

typical

data rate of 2 kbit/s. In addition, the

device contains a two tone warbler which functions
as a telephone ringer. The device is fabricated in
Zarlink's double-poly ISO

-CMOS technology

utilizing switched-capacitor techniques.

Ordering Information

MT8840AE

18 Pin Plastic DIP

MT8840AS

18 Pin SOIC

C to +85

Figure 1 - Functional Block Diagram

OSC1

OSC2

CK32

ETC MTC FATC

TxDI

RxDO

DET

CRx

Ref

LOOP

TCO

TxO

RxE

RxI

Timing

and

Control

Tone

Caller

Modulator

TX Bandpass

Filter

TX Post-

Filter

RX Pre-

Filter

R
O
U

AGC

RX Bandpass

Filter

Demodulator

ISSUE 4

November 1990

ISO

-CMOS

MT8840

ISO

-CMOS

5-4

Figure 2 - Pin Connections

Pin Description

Pin #

Name

Description

DET

Demodulator detection level adjust input (Analog). Internal resistor divider applies
2.36V in open circuit condition. Connection of external resistor will vary detect level.

CRx

External AGC time constant adjust input (Analog). Connect external capacitor to V

RxI

Modulated receive signal input (Analog). Biased at V

Ref

RxE

Receive enable input (Digital) with internal pull up. Active high.

LOOP

Self-test mode select input (Digital) with internal pull down. Active high.

Ref

Internal reference supply voltage input (Analog) .

TxO

Modulated transmit carrier output (Analog).

TxDI

Transmit data input (Digital).

Negative power supply.

FATC

Tone caller center frequency adjust input (Analog).

TCO

Tone caller output (Digital).

MTC

Mute tone caller input (Digital) with internal pull down. Active high.

ETC

Enable tone caller input (Digital) with internal pull down. Active high.

CK32

32 kHz data strobe output (Digital).

OSC1

Clock Input

3.579545 MHz crystal connected between
these pins completes internal oscillator.

OSC2

Clock Output to drive external devices.

RxDO

Receive data output (Digital). Synchronized to CK32.

Positive power supply.

1
2
3
4
5
6
7
8
9

18
17
16
15
14
13
12
11

DET

CRx

Rxl

RxE

LOOP

VRef

TxO

TxDI

VSS

VDD
RxDO
OSC2
OSC1
CK32
ETC
MTC
TCO
FATC

18 PIN PLASTIC DIP/SOIC

ISO

-CMOS

MT8840

5-5

Functional Description

The MT8840 contains the modulator and
demodulator circuitry for 32 kHz ASK signalling as
well as a two-tone warbler (tone caller) to replace the
function of the mechanical telephone ringer.

A 32 kHz carrier is 100% amplitude modulated by the
digital bit stream applied to input TxDI. This results in
an amplitude shift keyed (ASK) 32 kHz carrier. A
logical high at TxDI disables the carrier and a logical
low enables it. The digitally modulated waveform is
shaped by the Tx BANDPASS FILTER and smoothed
by the Tx POST FILTER. The signal then enters the
routing block where it is transferred to the TxO
output.

The modulated 32 kHz receive signal is applied to
RxI. With a logical low applied to LOOP and a logical
high applied to RxE, receive signals are routed to the
Rx PREFILTER. High frequencies are removed by
the Rx PREFILTER to prevent aliasing in the
switched capacitor Rx BANDPASS FILTER. Voice
signals are removed by the bandpass filter which is
followed by an AGC circuit. This provides a dynamic
range of 20dB for the receiver. An external 1

capacitor connected from CRx to V

is required to

control the AGC attack and decay time constants.
Data is recovered from the received signal in the
demodulator. The minimum voltage level to which the
demodulator responds may be adjusted by
connecting a resistor from DET to V

or V

Ref

. Since

DET is the input to a comparator, noise should be
kept to a minimum at this pin. The recovered receive
data is synchronized to the leading edge of the 32
kHz clock (available at CK32) before appearing at
RxDO.

When in loop around mode, the Rx PREFILTER input
is internally disconnected from the RxI input pin and
connected to TxO. The transmitter output is still
available at TxO.

A two tone warbling audio signal is available at TCO
when the tone caller enable input (ETC) is high. TCO
is internally clamped to V

Ref

when the tone caller is

disabled. The tone output can be attenuated by 20dB
if a logical high is applied to the tone caller mute
input (MTC).

Applications

Figures 3 through 5 show how the MT8840 may be
utilized to transfer data and voice simultaneously
over a single pair of wires in digital or analog PABXs
and "smart" telephone sets. In all three figures a
microprocessor sends/receives data to/from the
MT8840 via a UART which converts the data format
from parallel-to-serial or serial-to-parallel for the
transmit and receive directions, respectively. In the
receive direction the MT8840 has on-board filters to
reject voice-band signals leaving only the 32 kHz
carrier. This carrier is then demodulated to recover
the received data. In the transmit direction the data
to be sent is modulated and passed on to a summing
circuit which sums the modulated 32 kHz carrier and
voiceband signals for transmission over the
telephone line. In the PABX the Filter/Codec has
filters which reject the 32 kHz carrier from the
received composite voice and data signal allowing
only voiceband signals to pass through which are
then PCM encoded for digital switching. However, in
both the analog PABX and smart telephone set,
lowpass filters could be included to bandlimit the
received signal leaving only voice signals to be
passed on to the switch array or handset earpiece.

ISO

-CMOS

MT8840

5-7

Figure 5 - Smart Telephone Set Block Diagram

Figure 6 - Crystal Oscillator Connections for Driving Multiple MT8840s

Summing

Circuit

Telephone

line

MT8840

Micro-

processor

UART

Tx Data

Rx Data

Low-
Pass
Filter

Handset

(Optional)

2W/4W

Converter

When a single crystal is shared among a number of MT8840 devices, OSC1 and OSC2 should be a.c. coupled with a 1000

pF capacitor as shown above. This capacitor is not needed between the device with the crystal and the first driven device.

A capacitor should be used in the first stage whenever such a chain of devices is driven from a clock instead of a crystal. A

33pF capacitor should be connected between OSC1 and VSS to compensate for the load on OSC2.

OSC1

OSC2

MT8840

C=33pF

MT8840

OSC1

OSC2

C=1000pF

MT8840

OSC1

OSC2

C=1000pF

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