LAN9312 datasheet(86/458 Pages) SMSC

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High Performance Two Port 10/100 Managed Ethernet Switch with 32-Bit Non-PCI CPU Interface

Datasheet

Revision 1.2 (04-08-08)

SMSC LAN9312

DATASHEET

7.2.1.3

Scrambler and PISO

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 scrambler also

performs the Parallel In Serial Out conversion (PISO) of the data.

The seed for the scrambler is generated from the PHY address, ensuring that each PHY will have its

own scrambler sequence. For more information on PHY addressing, refer to Section 7.1.1, "PHY

Addressing".

7.2.1.4

NRZI and MLT-3 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 then encoded to MLT-3. MLT-3 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”.

7.2.1.5

100M Transmit Driver

The MLT-3 data is then passed to the analog transmitter, which drives the differential MLT-3 signal on

output pins TXPx and TXNx (where “x” is replaced with “1” for the Port 1 PHY, or “2” for the Port 2

PHY), 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

Ω impedance of the CAT-5 cable. Cable termination and impedance

matching require external components.

7.2.1.6

100M Phase Lock Loop (PLL)

The 100M PLL locks onto the reference clock and generates the 125MHz clock used to drive the 125

MHz logic and the 100BASE-TX Transmitter.

00010

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