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
MITEL [ MITEL NETWORKS CORPORATION ]
