Single-Chip Ethernet Controller with HP Auto-MDIX Support and PCI Interface

Datasheet

3.6.310BASE-T Transmit

Data to be transmitted comes from the MAC. The 10BASE-T transmitter receives 4-bit nibbles from the internal 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)

3.6.3.110M 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.

3.6.3.2Manchester 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 (internal TX_EN is low), the TX10M block outputs Normal Link Pulses (NLPs) to maintain communications with the remote link partner.

3.6.3.310M 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 TPO+ and TPO- outputs.

3.6.410BASE-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.

This 10M receiver uses the following blocks:

„Filter and SQUELCH (analog)

„10M PLL (analog)

„RX 10M (digital)

„MII (digital)

3.6.4.110M Receive Input and Squelch

The Manchester signal from the cable is fed into the PHY (on inputs TPI+ and TPI-) 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.

3.6.4.2Manchester 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 TPI+ is connected to TPI- of the remote partner and vice versa), then this is identified and corrected. The reversed condition

SMSC LAN9420/LAN9420i

69

Revision 1.22 (09-25-08)

 

DATASHEET