Datasheet

 2016 Microchip Technology Inc. DS00002199A-page 13
KSZ8081RNA/RND
3.2.1.1 Reference Clock (REF_CLK)
REF_CLK is a continuous 50 MHz clock that provides the timing reference for TXEN, TXD[1:0], CRS_DV, RXD[1:0],
and RX_ER.
For RMII – 25 MHz Clock Mode, the KSZ8081RNA/RND generates and outputs the 50 MHz RMII REF_CLK to the MAC
at REF_CLK (Pin 16).
For RMII – 50 MHz Clock Mode, the KSZ8081RNA/RND takes in the 50 MHz RMII REF_CLK from the MAC or system
board at XI (Pin 8) and leaves the REF_CLK (Pin 16) as no connect.
3.2.1.2 Transmit Enable (TXEN)
TXEN indicates that the MAC is presenting dibits on TXD[1:0] for transmission. It is asserted synchronously with the first
dibit of the preamble and remains asserted while all dibits to be transmitted are presented on the RMII. It is negated
before the first REF_CLK following the final dibit of a frame.
TXEN transitions synchronously with respect to REF_CLK.
3.2.1.3 Transmit Data[1:0] (TXD[1:0])
TXD[1:0] transitions synchronously with respect to REF_CLK. When TXEN is asserted, the PHY accepts TXD[1:0] for
transmission.
TXD[1:0] is 00 to indicate idle when TXEN is de-asserted. The PHY ignores values other than 00 on TXD[1:0] while
TXEN is de-asserted.
3.2.1.4 Carrier Sense/Receive Data Valid (CRS_DV)
The PHY asserts CRS_DV when the receive medium is non-idle. It is asserted asynchronously when a carrier is
detected. This happens when squelch is passed in 10 Mbps mode, and when two non-contiguous 0s in 10 bits are
detected in 100 Mbps mode. Loss of carrier results in the de-assertion of CRS_DV.
While carrier detection criteria are met, CRS_DV remains asserted continuously from the first recovered dibit of the
frame through the final recovered dibit. It is negated before the first REF_CLK that follows the final dibit. The data on
RXD[1:0] is considered valid after CRS_DV is asserted. However, because the assertion of CRS_DV is asynchronous
relative to REF_CLK, the data on RXD[1:0] is 00 until receive signals are properly decoded.
3.2.1.5 Receive Data[1:0] (RXD[1:0])
RXD[1:0] transitions synchronously with respect 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.
RXD[1:0] is 00 to indicate idle when CRS_DV is de-asserted. The MAC ignores values other than 00 on RXD[1:0] while
CRS_DV is de-asserted.
TABLE 3-1: RMII SIGNAL DEFINITION
RMII Signal
Name
Direction with
Respect to PHY,
KSZ8081 Signal
Direction with
Respect to MAC
Description
REF_CLK
Output (25 MHz
Clock Mode)/
No Connect (50 MHz
Clock Mode)
Input/
Input or No Connect
Synchronous 50 MHz reference clock for receive, trans-
mit, and control interface
TXEN Input Output Transmit Enable
TXD[1:0] Input Output Transmit Data[1:0]
CRS_DV Output Input Carrier Sense/Receive Data Valid
RXD[1:0] Output Input Receive Data[1:0]
RXER Output
Input or
Not Required
Receive Error