LAN8742AI-CZ datasheet(21/124 Pages) MICROCHIP

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DS00001989A-page 21

LAN8742A/LAN8742Ai

3.1.3.2

Manchester Encoding

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

10M PLL locks onto the external clock or internal oscillator and produces a 20 MHz clock. This is used to Manchester

encode the NRZ data stream. When no data is being transmitted (TXEN is low), the 10M TX block outputs Normal Link

Pulses (NLPs) to maintain communications with the remote link partner.

3.1.3.3

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 TXP and TXN outputs.

3.1.4

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 via MII at a rate of 2.5 MHz.

This 10M receiver uses the following blocks:

• Filter and SQUELCH (analog)

• 10M PLL (analog)

• RX 10M (digital)

• MII (digital)

3.1.4.1

10M Receive Input and Squelch

The Manchester signal from the cable is fed into the transceiver (on inputs RXP and RXN) 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 300 mV and detect and recognize

differential voltages above 585 mV.

3.1.4.2

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 XPOL bit of the Special Con-

trol/Status Indications Register. The 10M PLL is locked onto the received Manchester signal, from which the 20 MHz

cock is generated. Using this clock, the Manchester encoded data is extracted and converted to a 10 MHz NRZI data

stream. It is then converted from serial to 4-bit wide parallel data.

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

3.1.4.3

10M Receive Data Across the RMII Interface

The 2-bit data nibbles are sent to the RMII block. These data nibbles are valid on the rising edge of the RMII REF_CLK.

3.1.4.4

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 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.

As shown in Section 4.2.2, "Basic Status Register", the Jabber Detect bit indicates that a jabber condition was detected.

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