KS8721B/BT
Micrel
Collision: Whenever the line state is half-duplex and the transmitter and receiver are active at the same time, the KS8721B/
BT asserts its collision signal, which is asynchronous to any clock.
RMII (Reduced MII) Data Interface
RMII interface specifies a low pin count (Reduced) Media Independent Interface (RMII) intended for use between Ethernet
PHYs and Switch or Repeater ASICs. It is fully compliant with IEEE 802.3u [2].
This interface has the following characteristics:
• It is capable of supporting 10Mbps and 100Mbps data rates.
• A single clock reference is sourced from the MAC to PHY (or from an external source).
• It provides independent 2-bit wide (di-bit) transmit and receive data paths.
• It uses TTL signal levels, compatible with common digital CMOS ASIC processes.
RMII Signal Definition
Direction
Direction
Signal Name
(w/ respect to the PHY)
(w/ respect to the MAC)
Use
REF_CLK
Input
Input or Output
Synchronous clock reference for receive, transmit and
control interface
CRS_DV
RXD[1:0]
TX_EN
Output
Output
Input
Input
Carrier Sense/Receive Data Valid
Receive Data
Input
Output
Transit Enable
TXD[1:0]
RX_ER
Input
Output
Transit Data
Output
Input (Not Required)
Receive Error
Note 1. Unused MII signals, TXD[3:2], TXER need to tie to GND when RMII is using.
Reference Clock (REF_CLK)
REF_CLK is a continuous 50MHz clock that provides the timing reference for CRS_DV, RXD[1:0], TX_EN, TXD[1:0], and
RX_E. REF_CLK is sourced by the MAC or an external source. Switch implementations may choose to provide REF_CLK as
an input or an output depending on whether they provide a REF_CLK output or rely on an external clock distribution device.
Each PHY device shall have an input corresponding to this clock but may use a single clock input for multiple PHYs
implemented on a single IC.
Carrier Sense/Receive Data Valid (CRS_DV)
CRS_DV is asserted asynchronously on detection of carrier due to the criteria relevant to the operating mode. That is, in
10BASE-Tmode,whensquelchispassedorin100BASE-Xmodewhen2non-contiguouszeroesin10bitsaredetectedcarrier
is said to be detected.
Loss of carrier shall result in the de-assertion of CRS_DV synchronous to REF_CLK. So long as carrier criteria are being met,
CRS_DV shall remain asserted continuously from the first recovered di-bit of the frame through the final recovered di-bit and
shall be negated prior to the first REF_CLK that follows the final di-bit.
ThedataonRXD[1:0]isconsideredvalidonceCRS_DVisasserted.However,sincetheassertionofCRS_DVisasynchronous
relative to REF_CLK, the data on RXD[1:0] shall be “00” until proper receive signal decoding takes place (see definition of
RXD[1:0] behavior).
Receive Data [1:0] (RXD[1:0])
RXD[1:0] shall transition synchronously to REF_CLK. For each clock period in which CRS_DV is asserted, RXD[1:0] transfers
two bits of recovered data from the PHY. In some cases (e.g. before data recovery or during error conditions) a pre-determined
value for RXD[1:0] is transferred instead of recovered data. RXD[1:0] shall be “00” to indicate idle when CRS_DV is de-
asserted. Values of RXD[1:0] other than “00” when CRS_DV is de-asserted are reserved for out-of-band signalling (to be
defined). Values other than “00” on RXD[1:0] while CRS_DV is de-asserted shall be ignored by the MAC/repeater. Upon
assertion of CRS_DV, the PHY shall ensure that RXD[1:0]=00 until proper receive decoding takes place.
Transmit Enable (TX_EN)
Transmit Enable TX_EN indicates that the MAC is presenting di-bits on TXD[1:0] on the RMII for trans-mission. TX_EN shall
be asserted synchronously with the first nibble of the preamble and shall remain asserted while all di-bits to be transmitted are
presented to the RMII. TX_EN shall be negated prior to the first REF_CLK following the final di-bit of a frame. TX_EN shall
transition synchronously with respect to REF_CLK.
August 2003
13
KS8721B/BT
MICREL [ MICREL SEMICONDUCTOR ]
