MITEL MT8840AE, MT8840AS Datasheet

ISO2-CMOS



Data Over Voice Modem

MT8840

Features

• Performs ASK (amplitude shift keyed)
modulat ion an d dem odula tion

• 32 kHz carrier f reque ncy

• Up to 2 kbit/s full duplex data transfer rate

• On-chip oscillator

• On-chip tone caller for alerting functions

• Adjustab le to ne call er fre quenc ies

• Selectab le self-l oop tes t mod e

• 5V/2.5mA power supply

2

•ISO

-CMOS and switch ed capa cito r

technologies

•18 Pin DIP

Applications

• Simultaneous data and voice communication in
PABXs

• 2 kbit/s data modem

• "Smart" t eleph one set s

ISSUE 4 November 1990

Ordering Information

MT8840AE 18 Pin Plastic D IP
MT8840AS 18 Pin SOIC

0°C to +85°C

Descript io n

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
MITEL’s double-poly ISO
utilizing switched-capacitor techniques.

2

-CMOS technology

TxDI

RxDO

OSC1 OSC2

Modulator

Demodulator

DET CRx V

Figure 1 - Functional Block Diagram

AGC

CK32

Timing

and

Control

TX Bandpass

RX Bandpass

DD

Filter

Filter

V

SS

ETC

MTC FATC

Tone

Caller

TX Post-

Filter

RX Pre-

Filter

V

Ref

R
O
U
T

I
N
G

LOOP

TCO

TxO

RxE

RxI

5-3

MT8840 ISO

2

-CMOS

DET
CRx

RxE

LOOP

VRef

TxO

TxDI

VSS

1
2

3

Rxl

4
5

6
7
8
9

18 PIN PLASTIC DIP/SOIC

18
17
16
15
14
13
12
11
10

VDD
RxDO
OSC2
OSC1
CK32
ETC
MTC
TCO
FATC

Figure 2 - Pin Connections

Pin Description

Pin # Name Descripti on

1 DET Demodulat or detect ion level adj ust input (Ana log). Internal resistor divider applie s

2.36V in open circuit condition. Connect ion of external resistor will vary detect level.
2 CRx E xtern al AGC time constant adjust input (Analog). Connect external capacitor to V
3 RxI Modulated receive signal input (Analog). Biased at V
4 R xE Receive enable inpu t (Digital) wit h inte rnal pull up. Active high.
5 LOOP Self-test mode select input (Digit al) with interna l pull down. Active high.
6V

Ref

Internal reference supply voltage inp ut (Analog) .
7 TxO Modulate d transmit carrier output (Analog).
8 TxDI Transmit data input (Digital).
9V

SS

Negative power supply.

10 FATC Tone caller center frequency adjust inpu t (Analog).
11 TCO Tone caller output (Digital).
12 MTC Mute tone caller input (Digital) with internal pull down. Active high.
13 ETC Enab le tone caller in put (Digital ) with int ernal pull down. Active high.
14 CK32 32 kHz data strobe output (Digital).
15 OSC1 Clock Input 3.579545 MHz crystal connected between
16 OSC2 Clock Output to drive external devices.

these pins complete s internal oscillato r.

17 RxDO Receive data output (Digital). Synchronize d to CK32.
18 V

DD

Positive power supply.

Ref

.

SS

.

5-4

ISO

2

-CMOS MT8840

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 1µF
capacitor connected from CRx to V
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 VDD or V
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.

is required to

SS

. Since

Ref

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 o n to th e s wi tch a r ra y or h a ndset earpiece.

When in loop around mode, the Rx PREFILTER
input is internally disconnect ed 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
disabled. The tone output can be attenuated by 20dB
if a logical high is applied to the tone caller mute
input (MTC ).

when the tone caller is

Ref

5-5