Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support
Datasheet
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 2.5MHz.
This 10M receiver uses the following blocks:
Filter and SQUELCH (analog)
10M PLL (analog)
RX 10M (digital)
MII (digital)
3.1.4.1
3.1.4.2
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 300mV and detect and recognize differential voltages above 585mV.
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 Control/Status Indications Register. The 10M PLL
is locked onto the received Manchester signal, from which the 20MHz cock is generated. 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 10M RX block also detects valid 10Base-T IDLE signals - Normal Link Pulses (NLPs) - to maintain
the link.
3.1.4.3
3.1.4.4
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.
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 45ms. Once TXEN is deasserted, the logic resets the jabber condition.
As shown in Section 4.2.2, "Basic Status Register," on page 50, the Jabber Detect bit indicates that a
jabber condition was detected.
3.2
Auto-negotiation
The purpose of the auto-negotiation function is to automatically configure the transceiver to the
optimum link parameters based on the capabilities of its link partner. Auto-negotiation is a mechanism
for exchanging configuration information between two link-partners and automatically selecting the
highest performance mode of operation supported by both sides. Auto-negotiation is fully defined in
clause 28 of the IEEE 802.3 specification.
Once auto-negotiation has completed, information about the resolved link can be passed back to the
controller via the Serial Management Interface (SMI). The results of the negotiation process are
SMSC LAN8720A/LAN8720Ai
23
Revision 1.4 (08-23-12)
DATASHEET
SMSC [ SMSC CORPORATION ]
