LAN9250 datasheet(216/421 Pages) MICROCHIP

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LAN9250

DS00001913A-page 216

 2015 Microchip Technology Inc.

12.2.4.2

Manchester Encoding

The 4-bit wide data is sent to the 10M TX block. The nibbles are converted to a 10Mbps serial NRZI data stream. The

10M PLL produces a 20MHz clock. This is used to Manchester encode the NRZ data stream. When no data is being

transmitted (internal MII TXEN is low), the 10M TX Driver block outputs Normal Link Pulses (NLPs) to maintain commu-

nications with the remote link partner.

12.2.4.3

10M Transmit Drivers

The Manchester encoded data is sent to the analog transmitter where it is shaped and filtered before being driven out

as a differential signal across the TXPx and TXNx outputs.

12.2.5

10BASE-T RECEIVE

The 10BASE-T receiver gets the Manchester-encoded analog signal from the cable via the magnetics. It recovers the

receive clock from the signal and uses this clock to recover the NRZI data stream. This 10M serial data is converted to

4-bit data nibbles which are passed to the controller across the internal MII at a rate of 2.5MHz.

10BASE-T reception uses the following blocks:

• Filter and SQUELCH (analog)

• 10M PLL (analog)

• RX 10M (digital)

• MII (digital)

12.2.5.1

10M Receive Input and Squelch

The Manchester signal from the cable is fed into the transceiver (on inputs RXPx and RXNx) via 1:1 ratio magnetics. It

is first filtered to reduce any out-of-band noise. It then passes through a SQUELCH circuit. The SQUELCH is a set of

amplitude and timing comparators that normally reject differential voltage levels below 300mV and detect and recognize

differential voltages above 585mV.

12.2.5.2

Manchester Decoding

The output of the SQUELCH goes to the 10M RX block where it is validated as Manchester encoded data. The polarity

of the signal is also checked. If the polarity is reversed (local RXP is connected to RXN of the remote partner and vice

versa), the condition is identified and corrected. The reversed condition is indicated by the 10Base-T Polarity State

(XPOL) bit in PHY Special Control/Status Indication Register (PHY_SPECIAL_CONTROL_STAT_IND). The 10M PLL

is locked onto the received Manchester signal, from which the 20MHz clock is generated. Using this clock, the Man-

chester encoded data is extracted and converted to a 10MHz NRZI data stream. It is then converted from serial to 4-bit

wide parallel data.

The RX10M block also detects valid 10BASE-T IDLE signals - Normal Link Pulses (NLPs) - to maintain the link.

12.2.5.3

10M Receive Data Across the Internal MII Interface

For reception, the 4-bit data nibbles are sent to the MII MAC Interface block. These data nibbles are clocked to the con-

troller at a rate of 2.5 MHz.

12.2.5.4

Jabber Detection

Jabber is a condition in which a station transmits for a period of time longer than the maximum permissible packet length,

usually due to a fault condition, which results in holding the internal MII TXEN input for a long period. Special logic is

used to detect the jabber state and abort the transmission to the line, within 45 ms. Once TXEN is deasserted, the logic

resets the jabber condition.

The Jabber Detect bit in the PHY Basic Status Register (PHY_BASIC_STATUS) indicates that a jabber condition was

detected.

12.2.6

AUTO-NEGOTIATION

The purpose of the Auto-Negotiation function is to automatically configure the transceiver to the optimum link parame-

ters based on the capabilities of its link partner. Auto-Negotiation is a mechanism for exchanging configuration informa-

tion between two link-partners and automatically selecting the highest performance mode of operation supported by

both sides. Auto-Negotiation is fully defined in clause 28 of the IEEE 802.3 specification and is enabled by setting the

Auto-Negotiation Enable (PHY_AN) of the PHY Basic Control Register (PHY_BASIC_CONTROL).

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