mal network operations. There is one descriptor area
for receive activity, and there is a separate area for
transmit activity. The descriptor space contains relocat-
able pointers to the network frame data, and it is used
to transfer frame status from the Am79C978 controller
to the software. The buffer areas are locations that hold
frame data for transmission or that accept frame data
that has been received.
BASIC FUNCTIONS
System Bus Interface
The Am79C978 controller is designed to operate as a
bus master during normal operations. Some slave I/O
accesses to the Am79C978 controller are required in
normal operations as well. Initialization of the
Am79C978 controller is achieved through a combina-
tion of PCI Configuration Space accesses, bus slave
accesses, bus master accesses, and an optional read
of a serial EEPROM that is performed by the
Am79C978 controller. The EEPROM read operation is
performed through the 93C46 EEPROM interface. The
ISO 8802-3 (IEEE/ANSI 802.3) Ethernet Address may
reside within the serial EEPROM. Some controller con-
figuration registers may also be programmed by the
EEPROM read operation.
Network Interfaces
The Am79C978 controller provides all of the PHY layer
functions for 10 Mbps (10BASE-T) or 1 Mbps. The
Am79C978 controller supports both half-duplex and
full-duplex operation on the network MII interface.
Media Independent Interface
The Am79C978 controller fully supports the MII ac-
cording to the IEEE 802.3u standard. This Reconcilia-
tion Sublayer interface allows a variety of PHYs
(100BASE-TX, 100BASE-FX, 100BASE-T4,
100BASE-T2, 10BASE-T, etc.) to be attached to the
Am79C978 device without future upgrade problems.
The MII interface is a 4-bit (nibble) wide data path inter-
face 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 2.
The Address PROM, on-chip board-configuration reg-
isters, and the Ethernet controller registers occupy 32
bytes of address space. I/O and memory mapped I/O
accesses are supported. Base Address registers in the
PCI configuration space allow locating the address
space on a wide variety of starting addresses.
Software Interface
The software interface to the Am79C978 controller is
divided into three parts. One part is the PCI configura-
tion registers used to identify the Am79C978 controller
and to setup the configuration of the device. The setup
information includes the I/O or memory mapped I/O
base address, mapping of the Expansion ROM, and
the routing of the Am79C978 controller interrupt chan-
nel. This allows for a jumperless implementation.
MII Transmit Interface
The MII transmit clock is generated by the external
PHY and is sent to the Am79C978 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 Am79C978 con-
troller to the external PHY and is synchronous to the
rising edge of TX_CLK. The transmit process starts
when the Am79C978 controller asserts the TX_EN,
which indicates to the external PHY that the data on
TXD(3:0) is valid.
The second portion of the software interface is the di-
rect access to the I/O resources of the Am79C978 con-
troller. The Am79C978 controller occupies 32 bytes of
address space that must begin on a 32-byte block
boundary. The address space can be mapped into I/O
or memory space (memory mapped I/O). The I/O Base
Address Register in the PCI Configuration Space con-
trols the start address of the address space if it is
mapped to I/O space. The Memory Mapped I/O Base
Address Register controls the start address of the ad-
dress space if it is mapped to memory space. The 32-
byte address space is used by the software to program
the Am79C978 controller operating mode, to enable
and disable various features, to monitor operating sta-
tus, and to request particular functions to be executed
by the Am79C978 controller.
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 Am79C978 controller does not use this method of
signaling errors on the transmit side. The Am79C978
controller will invert the FCS on the last byte generating
an invalid FCS. The TX_ER pin should be tied to GND.
The third portion of the software interface is the de-
scriptor and buffer areas that are shared between the
software and the Am79C978 controller during normal
network operations. The descriptor area boundaries
are set by the software and do not change during nor-
Am79C978
33
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