LAN89218 datasheet(65/148 Pages) MICROCHIP

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DS60001255B-page 65

LAN89218

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 2.5 MHz and converts them to a 10 Mbps 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 2.5 MHz 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 20 MHz 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 2.5 MHz.

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 300 mV and detect and

recognize differential voltages above 585 mV.

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

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

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