MT8971B/72B
the Precan pin high. This mode simplifies the design
of external line transceivers used for loop extension
applications. The Precan pin features an internal
pull-down which allows this pin to be left
unconnected in applications where this function is
not required. The resultant signal passes through
a receive filter to bandlimit and equalize it. At this
point, the echo estimate from the echo canceller is
subtracted from the precancelled received signal.
This difference signal is then input to the echo
canceller as an error signal and also squared up by a
comparator and passed to the biphase receiver.
Within the echo canceller, the sign of this error signal
is determined. Depending on the sign, the echo
estimate is either incremented or decremented and
this new estimate is stored back in RAM.
In MOD mode, all the ports have a different format.
The line port again operates at 80 or 160 kbit/s,
however, there is no synchronization overhead, only
transparent data. The DV and CD ports carry serial
data at 80 or 160 kbit/s with the DV port transferring
all the data for the line and the CD port carrying the
C-channel only. In this mode the transfer of data at
both ports is synchronized to the TCK and RCK
clocks for transmit and receive data, respectively.
The CLD signal goes low to indicate the start of the
C-channel data on the CD port. It is used to load and
latch the input and output C-channel but has no
relationship to the data on the DV port.
Operating Modes (MS0-2)
The timebase in both SLV and MAS modes
(generated internally in SLV mode and externally in
MAS mode) is phase-locked to the received data
The logic levels present on the mode select pins
MS0, MS1 and MS2 program the DNIC for different
operating modes and configure the DV and CD ports
stream.
This phase-locked clock operates the
accordingly.
Table
1
shows the modes
Biphase Decoder, Descrambler and Deprescrambler
in MAS mode and the entire chip in SLV mode. The
Biphase Decoder decodes the received encoded bit
stream resulting in the original NRZ data which is
passed onto the Descrambler and Deprescrambler
where the data is restored to its original content by
performing the reverse polynomials. The SYNC bits
are extracted and the Receive Interface separates
the channels and outputs them to the proper ports in
the proper channel times. The destination of the
various channels is the same as that received on the
input DV and CD ports.
corresponding to the state of MS0-2. These pins
select the DNIC to operate as a MASTER or SLAVE,
in DUAL or SINGLE port operation, in MODEM or
DIGITAL NETWORK mode and the order of the C
and D channels on the CD port. Table 2 provides a
description of each mode and Table 3 gives a pin
configuration according to the mode selected for all
pins that have variable functions. These functions
vary depending on whether it is in MAS or SLV, and
whether DN or MOD mode is used.
The overall mode of operation of the DNIC can be
programmed to be either
a baseband modem
The Transmit/Receive Timing and Control block
generates all the clocks for the transmit and receive
functions and controls the entire chip according to
the control register. In order that more than one
DNIC may be connected to the same DV and
CD ports an F0o signal is generated which signals
the next device in a daisy chain that its channel
(MOD mode) or a digital network transceiver (DN
mode). As a baseband modem, transmit/receive
data is passed transparently through the device at 80
or 160 kbit/s by the DV port. The CD port transfers
the C-channel and D-Channel also at 80 or 160
kbit/s.
times are now active.
In this arrangement only
the first DNIC in the chain receives the system F0
with the following devices receiving its predecessor’s
F0o.
In DN mode, both the DV and CD ports operate as
ST-BUS streams at 2.048 Mbit/s. The DV port
transfers data over pins DSTi and DSTo while on the
Mode Select Pins
Mode
Operating Mode
MS2 MS1
MS0
SLV
MAS
E
DUAL SINGL MOD
DN
D-C
E
C-D
ODE
E
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
0
1
2
3
4
5
6
7
E
E
E
E
E
E
E
E
X
X
E
E
E
E
E
E
E
E
E
E
X
E
E
E
E
E
E
E
E
E
X
E
E
E
E
E
E
E
Table 1. Mode Select Pins
E=Enabled
X=Not Applicable
Blanks are disabled
9-113
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