SMSC LAN9312 - Manual (Page 87)

High Performance Two Port 10/100 Managed Ethernet Switch with 32-Bit Non-PCI CPU Interface
Datasheet
SMSC LAN9312 87 Revision 1.4 (08-19-08)
DATASHEET
7.2.2 100BASE-TX Receive
The 100BASE-TX receive data path is shown in Figure 7.3 . Shaded blocks are those which are internal
to the PHY. Each major block is explained in the following sections.
7.2.2.1 A/D Converter
The MLT-3 data from the cable is fed into the PHY on inputs RXPx and RXNx (where “x” is replaced
with “1” for the Port 1 PHY, or “2” for the Port 2 PHY) 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 quantizer, 6
digital bits are generated to represent each sample. The DSP adjusts the gain of the A/D Converter
(ADC) according to the observed signal levels such that the full dynamic range of the ADC can be
used.
7.2.2.2 DSP: Equalizer, BLW Correction and Clock/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 (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 PHY 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 7.3 100BASE-TX Receive Data Path
Port x
MAC
A/D
Converter
MLT-3
Converter
NRZI
Converter
4B/5B
Decoder
Magnetics CAT-5 RJ45
100M
PLL
Internal
MII 25MHz by 4 bits
Internal
MII Receive Clock
25MHz by
5 bits
NRZI
MLT-3 MLT-3 MLT-3
6 bit Data
Descrambler
and SIPO
125 Mbps Serial
DSP: Timing
recovery, Equalizer
and BLW Correction
MLT-3
MII MAC
Interface 25MHz
by 4 bits