APPENDIX C
Am79C973/Am79C975
268
Media Independent Interface (MII)
APPENDIX C: MEDIA INDEPENDENT INTERFACE (MII)
Introduction
The Am79C973/Am79C975 controller fully supports
the MII according to the IEEE 802.3 standard. This
Reconciliation Sublayer interface allows a variety of
PHYs (100BASE-TX, 100BASE-FX, 100BASE-T4,
100BASE-T2, 10BASE-T, etc.) to be attached to the
Am79C973/Am79C975 MAC engine without future up-
grade problems. The MII interface is a 4-bit (nibble)
wide data path interface that runs at 25 MHz for 100-
Mbps networks or 2.5 MHz for 10-Mbps networks. The
interface consists of two independent data paths, re-
ceive (RXD(3:0)) and transmit (TXD(3:0)), control sig-
nals for each data path (RX_ER, RX_DV, TX_ER,
TX_EN), network status signals (COL, CRS), clocks
(RX_CLK, TX_CLK) for each data path, and a two-wire
management interface (MDC and MDIO). See Figure
C-77.
Note:
The MII interface is disabled by default. It is en-
abled by setting PHYSELEN (BCR2 bit 13) = 1 and
PHYSEL (BCR18 bit 4, 3) =10.
Enabling the MII interface automatically disables the in-
ternal 10/100 PHY and the Expansion Bus. When in
this mode, the Am79C973/Am79C975 MII interface
pins are multiplexed with the expansion bus pins. Refer
to the connection diagram showing how the pins are
multiplexed.
MII Transmit Interface
The MII transmit clock is generated by the external
PHY and is sent to the Am79C973/Am79C975 control-
ler on the TX_CLK input pin. The clock can run at 25
MHz or 2.5 MHz, depending on the speed of the net-
work to which the external PHY is attached. The data
is a nibble-wide (4 bits) data path, TXD(3:0), from the
Am79C973/Am79C975 controller to the external PHY
and is synchronous to the rising edge of TX_CLK. The
transmit process starts when the Am79C973/
Am79C975 controller asserts the TX_EN, which indi-
cates to the external PHY that the data on TXD(3:0) is
valid.
Normally, unrecoverable errors are signaled through
the MII to the external PHY with the TX_ER output pin.
The external PHY will respond to this error by generat-
ing a TX coding error on the current transmitted frame.
The Am79C973/Am79C975 controller does not use
this method of signaling errors on the transmit side.
The Am79C973/Am79C975 controller will invert the
FCS on the last byte generating an invalid FCS. The
TX_ER pin is reserved for future use and is actively
driven to 0.
Figure 77. Media Independent Interface
4
RXD(3:0)
RX_DV
RX_ER
RX_CLK
CRS
4
TXD(3:0)
TX_EN
Am79C973/Am79C975
M
COL
Receive Signals
Transmit Signals
Management Port Signals
Network Status Signals
TX_CLK
MDIO
MDC
21510C-77