LAN9220 datasheet(61/142 Pages) MICROCHIP

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DS00002417A-page 61

LAN9220

4.4

10Base-T Transmit

Data to be transmitted comes from the MAC layer controller. The 10Base-T transmitter receives 4-bit nibbles from the

MII at a rate of 2.5MHz and converts them to a 10Mbps serial data stream. The data stream is then Manchester-encoded

and sent to the analog transmitter, which drives a signal onto the twisted pair via the external magnetics.

The 10M transmitter uses the following blocks:

• MII (digital)

• TX 10M (digital)

• 10M Transmitter (analog)

• 10M PLL (analog)

4.4.1

10M TRANSMIT DATA ACROSS THE INTERNAL MII BUS

The MAC controller drives the transmit data onto the internal TXD BUS. When the controller has driven TX_EN high to

indicate valid data, the data is latched by the MII block on the rising edge of TX_CLK. The data is in the form of 4-bit

wide 2.5MHz data.

4.4.2

MANCHESTER ENCODING

The 4-bit wide data is sent to the TX10M block. The nibbles are converted to a 10Mbps serial NRZI data stream. The

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

encode the NRZ data stream. When no data is being transmitted (TX_EN is low), the TX10M block outputs Normal Link

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

4.4.3

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 TXP and TXN outputs.

4.5

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 MII at a rate of 2.5MHz.

This 10M receiver uses the following blocks:

• Filter and SQUELCH (analog)

• 10M PLL (analog)

• RX 10M (digital)

• MII (digital)

4.5.1

10M RECEIVE INPUT AND SQUELCH

The Manchester signal from the cable is fed into the PHY (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 300mV and detect and recognize differential

voltages above 585mV.

4.5.2

MANCHESTER DECODING

The output of the SQUELCH goes to the RX10M 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), then this is identified and corrected. The reversed condition is indicated by the flag “XPOL“, bit 4 in register 27.

The 10M PLL is locked onto the received Manchester signal and from this, generates the received 20MHz clock. Using

this clock, the Manchester 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.

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