®
ISO
2
-CMOS
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
2
-CMOS and switched capacitor
technologies
18 Pin DIP
MT8840AE
MT8840AS
ISSUE 4
November 1990
Ordering Information
18 Pin Plastic DIP
18 Pin SOIC
0°C to +85°C
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
MITEL’s
double-poly
ISO
2
-CMOS
technology
utilizing switched-capacitor techniques.
Applications
•
•
•
Simultaneous data and voice communication in
PABXs
2 kbit/s data modem
"Smart" telephone sets
OSC1
OSC2
CK32
ETC MTC FATC
Timing
and
Control
Tone
Caller
TCO
TxDI
Modulator
TX Bandpass
Filter
TX Post-
Filter
RxDO
Demodulator
AGC
RX Bandpass
Filter
RX Pre-
Filter
R
O
U
T
I
N
G
TxO
RxE
RxI
DET
CRx
V
DD
V
SS
V
Ref
LOOP
Figure 1 - Functional Block Diagram
5-3
MT8840
ISO
2
-CMOS
DET
CRx
Rxl
RxE
LOOP
VRef
TxO
TxDI
VSS
1
2
3
4
5
6
7
8
9
18
17
16
15
14
13
12
11
10
VDD
RxDO
OSC2
OSC1
CK32
ETC
MTC
TCO
FATC
18 PIN PLASTIC DIP/SOIC
Figure 2 - Pin Connections
Pin Description
Pin #
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Name
DET
CRx
RxI
RxE
LOOP
V
Ref
TxO
TxDI
V
SS
FATC
TCO
MTC
ETC
CK32
OSC1
OSC2
RxDO
V
DD
Description
Demodulator detection level adjust input (Analog). Internal resistor divider applies
2.36V in open circuit condition. Connection of external resistor will vary detect level.
External AGC time constant adjust input (Analog). Connect external capacitor to V
SS
.
Modulated receive signal input (Analog). Biased at V
Ref
.
Receive enable input (Digital) with internal pull up. Active high.
Self-test mode select input (Digital) with internal pull down. Active high.
Internal reference supply voltage input (Analog) .
Modulated transmit carrier output (Analog).
Transmit data input (Digital).
Negative power supply.
Tone caller center frequency adjust input (Analog).
Tone caller output (Digital).
Mute tone caller input (Digital) with internal pull down. Active high.
Enable tone caller input (Digital) with internal pull down. Active high.
32 kHz data strobe output (Digital).
Clock Input
Clock Output to drive external devices.
Positive power supply.
3.579545 MHz crystal connected between
these pins completes internal oscillator.
Receive data output (Digital). Synchronized to CK32.
5-4
ISO
2
-CMOS
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
SS
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
DD
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).
MT8840
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.
5-5
Test/Measurement
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