LAN9420 Datasheet, PDF (70/171 Pages) SMSC Corporation – Single-Chip Ethernet Controller with HP Auto-MDIX Support and PCI Interface

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Single-Chip Ethernet Controller with HP Auto-MDIX Support and PCI Interface
Datasheet
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.
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
(internal TX_EN is low), the TX10M block outputs Normal Link Pulses (NLPs) to maintain
communications with the remote link partner.
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 TPO+ and TPO- outputs.
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.
This 10M receiver uses the following blocks:
„ Filter and SQUELCH (analog)
„ 10M PLL (analog)
„ RX 10M (digital)
„ MII (digital)
10M 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.
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 TPI+ is connected to
TPI- 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.
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, that results in holding the internal TX_EN
input for a long period. Special logic is used to detect the jabber state and abort the transmission to
Revision 1.6 (07-18-11)
70
DATASHEET
SMSC LAN9420/LAN9420i