lan8720 Standard Microsystems Corp., lan8720 Datasheet - Page 20

• Manufacturer Part Number: lan8720
• Description: Small Footprint Rmii 10/100 Ethernet Transceiver With Hp Auto-mdix Support
• Manufacturer: Standard Microsystems Corp.

Revision 1.0 (04-15-09)
4.2.3
4.2.4
4.2.5
GROUP
CODE
00100
00110
11001
00000
00001
00010
00011
00101
01000
01100
10000
00111
Scrambling
Repeated data patterns (especially the IDLE code-group) can have power spectral densities with large
narrow-band peaks. Scrambling the data helps eliminate these peaks and spread the signal power
more uniformly over the entire channel bandwidth. This uniform spectral density is required by FCC
regulations to prevent excessive EMI from being radiated by the physical wiring.
The seed for the scrambler is generated from the transceiver address, PHYAD[4:0], ensuring that in
multiple-transceiver applications, such as repeaters or switches, each transceiver will have its own
scrambler sequence.
The scrambler also performs the Parallel In Serial Out conversion (PISO) of the data.
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”.
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 100Ω impedance of the CAT-5 cable. Cable termination
and impedance matching require external components.
SYM
R
H
V
V
V
V
V
V
V
V
V
V
Second nibble of ESD, causes
deassertion of CRS if following /T/, else
assertion of RXER
Transmit Error Symbol
INVALID, RXER if during RXDV
INVALID, RXER if during RXDV
INVALID, RXER if during RXDV
INVALID, RXER if during RXDV
INVALID, RXER if during RXDV
INVALID, RXER if during RXDV
INVALID, RXER if during RXDV
INVALID, RXER if during RXDV
INVALID, RXER if during RXDV
INVALID, RXER if during RXDV
Table 4.1 4B/5B Code Table (continued)
INTERPRETATION
RECEIVER
DATASHEET
20
Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support
Sent for falling TXEN
Sent for rising TXER
INVALID
INVALID
INVALID
INVALID
INVALID
INVALID
INVALID
INVALID
INVALID
INVALID
INTERPRETATION
TRANSMITTER
SMSC LAN8720/LAN8720i
Datasheet