KS8721B/BT
Micrel
Transmit Data [1:0] (TXD[1:0])
Transmit Data TXD[1:0] shall transition synchronously with respect to REF_CLK. When TX_EN is asserted, TXD[1:0] are
accepted for transmission by the PHY. TXD[1:0] shall be “00” to indicate idle when TX_EN is de-asserted. Values of TXD[1:0]
other than “00” when TX_EN is de-asserted are reserved for out-of-band signalling (to be defined). Values other than “00” on
TXD[1:0] while TX_EN is deasserted shall be ignored by the PHY.
Collision Detection
Since the definition of CRS_DV and TX_EN both contain an accurate indication of the start of frame, the MAC can reliably
regenerate the COL signal of the MII by ANDing TX_EN and CRS_DV.
During the IPG time following the successful transmission of a frame, the COL signal is asserted by some transceivers as a
self-test. The Signal Quality Error (SQE) function will not be supported by the reduced MII due to the lack of the COL signal.
Historically, SQE was present to indicate that a transceiver located physically remote from the MAC was functioning. Since
the reduced MII only supports chip-to-chip connections on a PCB, SQE functionality is not required.
RX_ER
The PHY shall provide RX_ER as an output according to the rules specified in IEEE 802.3u [2] (see Clause 24, Figure 24-11
- Receive State Diagram). RX_ER shall be asserted for one or more REF_CLK periods to indicate that an error (e.g. a coding
error or any error that a PHY is capable of detecting, and that may otherwise be undetectable by the MAC sublayer) was
detectedsomewhereintheframepresentlybeingtransferredfromthePHY.RX_ERshalltransitionsynchronouslywithrespect
to REF_CLK. While CRS_DV is de-asserted, RX_ER shall have no effect on the MAC.
RMII AC Characteristics
Symbol
Parameter
Min
Typ
Max
Units
MHz
%
REF_CLK Frequency
REF_CLK Duty Cycle
50
35
4
65
tSU
tH
TXD[1:0]. TX_EN, RXD[1:0], CRS_DV,
RX_ER Data Set-Up to REF_CLK Rising
ns
TXD[1:0]. TX_EN, RXD[1:0], CRS_DV,
RXER Data Hold from REF_CLK
Rising Edge
2
ns
KS8721B/BT
14
August 2003
MICREL [ MICREL SEMICONDUCTOR ]
