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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support
Datasheet
SMSC LAN8720/LAN8720i 21 Revision 1.0 (05-28-09)
DATASHEET
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.
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 transceiver (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.
4.3.2 Equalizer, Baseline Wander Correction and Clock and Data Recovery
The 6 bits from the ADC are fed into the DSP block. The equalizer in the DSP section compensates
for phase and amplitude distortion caused by the physical channel consisting of magnetics, connectors,
and CAT- 5 cable. The equalizer can restore the signal for any good-quality CAT-5 cable between 1m
and 150m.
If the DC content of the signal is such that the low-frequency components fall below the low frequency
pole of the isolation transformer, then the droop characteristics of the transformer will become
significant and Baseline Wander (BLW) on the received signal will result. To prevent corruption of the
received data, the transceiver corrects for BLW and can receive the ANSI X3.263-1995 FDDI TP-PMD
defined “killer packet” with no bit errors.
The 100M PLL generates multiple phases of the 125MHz clock. A multiplexer, controlled by the timing
unit of the DSP, selects the optimum phase for sampling the data. This is used as the received
recovered clock. This clock is used to extract the serial data from the received signal.
Figure 4.2 Receive Data Path
MAC
Tx
Driver
MLT-3
Converter
NRZI
Converter
4B/5B
Encoder
CAT-5RJ45
25MHz by
5 bits
NRZI
MLT-3MLT-3
MLT-3
Scrambler
and PISO
RMII
25MHz
by 4 bits
Ref_CLK
PLL
RMII 50Mhz by 2 bits
MLT-3
Magnetics
125 Mbps Serial