LAN8700 Datasheet, PDF (16/74 Pages) SMSC Corporation – ±15kV ESD Protected MII/RMII Fast-Ethernet PHY with HP Auto-MDIX and SMSC flexPWR TM in a Small Footprint

LAN8700/LAN8700i
4.2.4 NRZI AND MLT3 ENCODING
The scrambler block passes the 5-bit wide parallel data to the NRZI converter where it becomes a serial 125MHz NRZI
data stream. The NRZI is encoded to MLT-3. MLT3 is a tri-level code where a change in the logic level represents a code
bit “1” and the logic output remaining at the same level represents a code bit “0”.
4.2.5 100M TRANSMIT DRIVER
The MLT3 data is then passed to the analog transmitter, which drives the differential MLT-3 signal, on outputs TXP and
TXN, to the twisted pair media across a 1:1 ratio isolation transformer. The 10Base-T and 100Base-TX signals pass
through the same transformer so that common “magnetics” can be used for both. The transmitter drives into the 100
impedance of the CAT-5 cable. Cable termination and impedance matching require external components.
4.2.6 100M PHASE LOCK LOOP (PLL)
The 100M PLL locks onto reference clock and generates the 125MHz clock used to drive the 125 MHz logic and the
100Base-Tx Transmitter.
FIGURE 4-2:
RECEIVE DATA PATH
MAC
RX_CLK
(for MII only)
100M
PLL
Ext Ref_CLK (for RMII only)
MII 25Mhz by 4 bits
or
RMII 50Mhz by 2 bits
MII/RMII
25MHz
by 4 bits
4B/5B
Decoder
125 Mbps Serial
25MHz by
5 bits
Descrambler
and SIPO
NRZI
Converter
NRZI
MLT-3
Converter
MLT-3
DSP: Timing
recovery, Equalizer
and BLW Correction
A/D
Converter
MLT-3
Magnetics
MLT-3
RJ45
MLT-3 CAT-5
6 bit Data
4.3 100Base-TX Receive
The receive data path is shown in Figure 4-2. Detailed descriptions are given below.
4.3.1 100M RECEIVE INPUT
The MLT-3 from the cable is fed into the PHY (on inputs RXP and RXN) via a 1:1 ratio transformer. The ADC samples
the incoming differential signal at a rate of 125M samples per second. Using a 64-level quanitizer it generates 6 digital
bits to represent each sample. The DSP adjusts the gain of the ADC according to the observed signal levels such that
the full dynamic range of the ADC can be used.
DS00002260A-page 16
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