
8/01/00
Am79C976
81
P R E L I M I N A R Y
—
Transmission is not deferred while receive is
active.
—
The IPG counter which governs transmit deferral
during the IPG between back-to-back transmits
is started when transmit activity for the first
packet ends, instead of when transmit and car-
rier activity ends.
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The Am79C976 controller provides bits in each of the
LED Status registers (BCR4, BCR5, BCR6, BCR7) to
display the Full-Duplex Link Status. If the FDLSE bit (bit
8) is set, a value of 1 will be sent to the associated LED-
OUT bit when in Full-Duplex.
Media Independent Interface
The Am79C976 controller fully supports the MII
according to the IEEE 802.3 standard. This Reconcili-
ation Sublayer interface allows a variety of PHYs
(100BASE-TX, 100BASE-FX, 100BASE-T4,
100BASE-T2, 10BASE-T, etc.) to be attached to the
Am79C976 MAC engine without future upgrade prob-
lems. 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, receive (RXD(3:0)) and
transmit (TXD(3:0)), control signals for each data path
(RX_ER, RX_DV, 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 3333.
The transmit and receive paths in the Am79C976 con-
troller's MAC are independent. The TX_CLK and
RX_CLK need not run at the same frequency. TX_CLK
can slow down or stop without affecting receive and
vice versa. It is only necessary to respect the minimum
clock high and low time specifications when switching
TX_CLK or RX_CLK. This facilitates operation with
PHYs that use MII signaling but do not adhere to 802.3
MII specifications.
#!(
The MII transmit clock is generated by the external
PHY and is sent to the Am79C976 controller on the
TX_CLK input pin. The clock can run at 25 MHz or 2.5
MHz, depending on the speed of the network to which
the external PHY is attached. The data is a nibble-wide
(4 bits) data path, TXD(3:0), from the Am79C976 con-
troller to the external PHY and is synchronous with the
rising edge of TX_CLK. The transmit process starts
when the Am79C976 controller asserts TX_EN, which
indicates to the external PHY that the data on TXD(3:0)
is valid.
IEEE Std 802.3 provides a mechanism for signalling
unrecoverable errors through the MII to the external
PHY with the TX_ER output pin. The external PHY will
respond to this error by generating a TX coding error on
the current transmitted frame. The Am79C976 control-
ler does not use this method of signaling errors on the
transmit side. Instead if the Am79C976 controller de-
tects a transmit error, it will invert the FCS to generate
an invalid FCS. Since the Am79C976 controller does
not implement the TX_ER function, the TX_ER pin on
the external PHY device should be connected to VSS.
#((
The MII receive clock is also generated by the external
PHY and is sent to the Am79C976 controller on the
RX_CLK input pin. The clock will be the same fre-
quency as the TX_CLK but will be out of phase and can
run at 25 MHz or 2.5 MHz, depending on the speed of
the network to which the external PHY is attached.