April 2005 13 M9999-041405
KS8721CL Micrel, Inc.
asserted. For 10BASE-T links, CRS assertion is based on reception of valid preamble, and de-assertion on reception of an
end-of-frame (EOF) marker.
Collision: Whenever the line state is half-duplex and the transmitter and receiver are active at the same time, the KS8721CL
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 Output Input Carrier Sense/Receive Data Valid
RXD[1:0] Output Input Receive Data
TX_EN Input Output Transmit Enable
TXD[1:0] Input Output Transmit Data
RX_ER Output Input (Not Required) Receive Error
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 must 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-T mode, when squelch is passed or in 100BASE-X mode when 2 noncontiguous zeroes in 10 bits are detected, the
carrier is detected.
Loss-of-carrier results in the de-assertion of CRS_DV synchronous to REF_CLK. As carrier criteria are met, CRS_DV remains
continuously asserted from the first recovered di-bit of the frame through the final recovered di-bit and is negated prior to the
first REF_CLK that follows the final di-bit.
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] remains as “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] transitions 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 predetermined
value for RXD[1:0] is transferred instead of recovered data. RXD[1:0] remains as “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 are ignored by the MAC/repeater. Upon assertion
of CRS_DV, the PHY ensures 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 transmission. TX_EN is
asserted synchronously with the first nibble of the preamble and remains asserted while all transmitted di-bits are presented
