LAN8720AI datasheet(27/79 Pages) SMSC

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet

SMSC LAN8720A/LAN8720Ai

Revision 1.4 (08-23-12)

DATASHEET

3.4

MAC Interface

3.4.1

RMII

The device supports the low pin count Reduced Media Independent Interface (RMII) intended for use

between Ethernet transceivers and switch ASICs. Under IEEE 802.3, an MII comprised of 16 pins for

data and control is defined. In devices incorporating many MACs or transceiver interfaces such as

switches, the number of pins can add significant cost as the port counts increase. RMII reduces this

pin count while retaining a management interface (MDIO/MDC) that is identical to MII.

The RMII interface has the following characteristics:

It is capable of supporting 10Mbps and 100Mbps data rates

A single clock reference is used for both transmit and receive

It provides independent 2-bit (di-bit) wide transmit and receive data paths

It uses LVCMOS signal levels, compatible with common digital CMOS ASIC processes

The RMII includes the following interface signals (1 optional):

transmit data - TXD[1:0]

transmit strobe - TXEN

receive data - RXD[1:0]

receive error - RXER (Optional)

carrier sense - CRS_DV

Reference Clock - (RMII references usually define this signal as REF_CLK)

3.4.1.1

CRS_DV - Carrier Sense/Receive Data Valid

The CRS_DV is asserted by the device when the receive medium is non-idle. CRS_DV is asserted

asynchronously on detection of carrier due to the criteria relevant to the operating mode. In 10BASE-

T mode when squelch is passed, or in 100BASE-X mode when 2 non-contiguous zeroes in 10 bits are

detected, the carrier is said to be detected.

Loss of carrier shall result in the deassertion of CRS_DV synchronous to the cycle of REF_CLK which

presents the first di-bit of a nibble onto RXD[1:0] (i.e. CRS_DV is deasserted only on nibble

boundaries). If the device has additional bits to be presented on RXD[1:0] following the initial

deassertion of CRS_DV, then the device shall assert CRS_DV on cycles of REF_CLK which present

the second di-bit of each nibble and de-assert CRS_DV on cycles of REF_CLK which present the first

di-bit of a nibble. The result is, starting on nibble boundaries, CRS_DV toggles at 25 MHz in 100Mbps

mode and 2.5 MHz in 10Mbps mode when CRS ends before RXDV (i.e. the FIFO still has bits to

transfer when the carrier event ends). Therefore, the MAC can accurately recover RXDV and CRS.

During a false carrier event, CRS_DV shall remain asserted for the duration of carrier activity. The data

on RXD[1:0] is considered valid once CRS_DV is asserted. However, since the assertion of CRS_DV

is asynchronous relative to REF_CLK, the data on RXD[1:0] shall be “00” until proper receive signal

decoding takes place.

3.4.1.2

Reference Clock (REF_CLK)

The RMII REF_CLK is a continuous clock that provides the timing reference for CRS_DV, RXD[1:0],

TXEN, TXD[1:0] and RXER. The device uses REF_CLK as the network clock such that no buffering

is required on the transmit data path. However, on the receive data path, the receiver recovers the

clock from the incoming data stream, and the device uses elasticity buffering to accommodate for

differences between the recovered clock and the local REF_CLK.

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