LAN9500AI datasheet(71/213 Pages) MICROCHIP

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DS00001875A-page 71

LAN950x

5.6.1.2

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

5.6.1.3

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”.

5.6.1.4

100M Transmit Driver

The MLT3 data is then passed to the analog transmitter, which launches the differential MLT-3 signal, on outputs TXP

and TXN, to the twisted pair media via 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.

5.6.1.5

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.

5.6.2

100BASE-TX RECEIVE

The receive data path is shown in Figure 5-18. Detailed descriptions are given in the following subsections.

11101

1111

11111

IDLE

Sent after /T/R until TX_EN

11000

First nibble of SSD, translated to “0101”

following IDLE, else RX_ER

Sent for rising TX_EN

10001

Second nibble of SSD, translated to

“0101” following J, else RX_ER

Sent for rising TX_EN

01101

First nibble of ESD, causes de-assertion

of CRS if followed by /R/, else assertion of

RX_ER

Sent for falling TX_EN

00111

Second nibble of ESD, causes

deassertion of CRS if following /T/, else

assertion of RX_ER

Sent for falling TX_EN

00100

Transmit Error Symbol

Sent for rising TX_ER

00110