Mitel NWK954 - Manual (Page 8)

NWK954
8
100BASE-TX Receiver
The 100BASE-TX receiver recovers data from up to 140m
of Cat5 UTP cable. Received data is decoded and descrambled
and presented to the repeater controller as 5-bit symbols. The
Transceiver Controller sequences the start-up of the receiver
and does not allow data to be passed to the Repeater Controller
until the receiver is fully initialized and a link is established and
the descrambler is synchronized. After start-up the Transceiver
Controller monitors the receiver and takes corrective action if a
fault is detected.
The Signal Detect continuously monitors the level on the
RXIP/RXIN differential input and indicates to the Transceiver
Controller when the signal amplitude is within the range of the
Equalizer. The acceptable level is considerably less than that
specified in the 802.3 Standard because the NWK954 receiver
is designed for recovery of signals from up to 140m of Cat5
UTP cable.
The Equalizer compensates for the signal attenuation and
distortion resulting from transmission down the cable and through
the isolation transformers. The Equalizer self- adjusts within 1ms
of Signal Detect indicating that the incoming signal is within the
acceptable range. Thereafter the Equalizer continuously adjusts
to small variations in signal level without corrupting the received
data.
The 100BASE-TX MLT3 code contains significant low
frequency components which are not passed through the
isolation transformers and cannot be restored by the Equalizer.
This leads to a phenomenon known as baseline wander (BLW)
which will cause an unacceptable increase in error rate if not
corrected. The NWK954 employs a quantized feedback
technique to restore the low frequency components and thus
maintain a very low error rate even when receiving signals such
as the ‘killer packet’ described in the TP-PMD specification.
The Clock Recovery circuit uses a Phase-Locked Loop (PLL)
to derive a sampling clock from the incoming signal. The
recovered clock runs at the symbol bit rate (nominally 125MHz)
and is used to clock the MLT3 decoder and the Serial-to-Parallel
converter (SIPO). The recovered clock is divided by 5 to generate
the receive clock which is used to strobe received data into the
Repeater Controller. The Transceiver Controller monitors
behaviour of the PLL and re-initializes the receiver if lock is lost.
The SIPO and Decoder convert the received signal from serial
MLT3 to 5-bit parallel NRZ.
The Link Monitor implements the 802.3 Link Monitor State
Machine which indicates when a sustained signal of appropriate
quality and amplitude is being received. This is the first stage in
establishing a link; no data can be passed to the Repeater
Controller until the Descrambler is synchronized to the incoming
signal. Descrambler synchronization is established during
reception of the idle pattern. After synchronization is established,
the Descrambler output is continuously monitored and the
Descrambler is re-synchronized if insufficient idle sequences
are detected.
100BASE-TX Transmitter
The 100BASE-TX transmitter generates a 125MHz transmit
clock and uses it to serialize and transmit the 5-bit symbol data
input from the Repeater Controller. The Transceiver Controller
sequences the start-up of the transmitter and does not allow
transmission onto the twisted pair until the transmitter is fully
initialized. After start-up the Transceiver Controller monitors the
transmitter and takes corrective action if a fault is detected.
The Scrambler mixes the symbol data with a 2047-bit pseudo-
random code, in accordance with the TP-PMD Standard. The
four Scramblers in the NWK954 are seeded with different values
based on the TA[4:2] input. When multiple NWK954s are
cascaded to make a hub, each NWK954 should have a unique
value on TA[4:2] to ensure that all of the Scramblers in the hub
are seeded with different values.
The 125MHz Synthesizer employs a phase-locked loop (PLL)
to generate a 125MHz timing reference from the 25MHz
reference clock. The Transceiver Controller monitors
behaviour of the PLL and re-initializes the Synthesizer if lock
is lost.
The PISO and Encoder take NRZ-coded symbols from
the Scrambler, and convert them to serial MLT3 for outputting
to the TX Driver. The PISO and Encoder do not operate until
the 125MHz Synthesizer is locked to the 25MHz reference.
This avoids transmission of spurious signals onto the twisted
pair.
The TX Driver outputs the differential signal onto the TXOP
and TXON pins. It operates with 1:1 magnetics to provide
impedance matching and amplification of the signal in
accordance with the 802.3 specifications. The transmit current
is governed by the current through the TXREF100 pin, which
must be grounded through a resistor as described in
Table 10.
Power Saving on unused ports
The NWK954 incorporates a feature that will automatically
shutdown the transceivers on unused ports. The shutdown
occurs if Signal Detect indicates that no signal has been received
for 2·5s. The transceiver is re-started when Signal Detect
indicates that an incoming signal has been detected. This feature
is intended to save power and reduce noise in unconnected
ports. In certain circumstances, such as in port-to-port links
between hubs, this feature should be suppressed by appropriate
setting of the PSEN [1:0] inputs, as shown in Table 3.
Initialization
The NWK954 incorporates a power-on reset circuit for self-
initialization on power-up. During power-on reset the open drain
RESET_N pin is driven low. It will not normally be necessary for
the user to drive RESET_N because the NWK954 is designed
to automatically recover from fault conditions; however, if
required, the user may initialize the device by pulsing RESET_N
low.
Function
Power saving disabled on all ports
Power saving enabled on ports 1, 2 and 3, disabled on port 0
Power saving enabled on ports 0, 1 and 2, disabled on port 3
Power saving enabled on all ports
PSEN1
0
0
1
1
PSEN0
0
1
0
1
Table 3 Power saving functions