AM79C976 Advanced Micro Devices, AM79C976 Datasheet - Page 37

• Manufacturer Part Number: AM79C976
• Description: PCnet-PRO 10/100 Mbps PCI Ethernet Controller
• Manufacturer: Advanced Micro Devices

The Am79C976 device includes an interface to an op-
tional expansion ROM. The amount of PCI address
space claimed by this ROM is determined by the con-
tents of the ROM Configuration Register, ROM_CFG,
which should normally be loaded from the serial EE-
PROM.
The host must initialize the Expansion ROM Base
Address register at offset 30H in the PCI configuration
space with a valid address before enabling the access
to the device. The Am79C976 controller will not react to
any access to the Expansion ROM until both MEMEN
(PCI Command register, bit 1) and ROMEN (PCI Ex-
pansion ROM Base Address register, bit 0) are set to 1.
After the Expansion ROM is enabled, the Am79C976
controller will assert DEVSEL on all memory read ac-
cesses to the memory space defined by the contents of
the Expansion ROM Base Address register. The
Am79C976 controller aliases all accesses to the Ex-
pansion ROM of the command types Memory Read
Multiple and Memory Read Line to the basic Memory
Read command. Eight-bit, 16-bit, and 32-bit read trans-
fers are supported.
Since setting MEMEN also enables memory mapped
access to the I/O resources, attention must be given
the PCI Memory Mapped I/O Base Address register
before enabling access to the Expansion ROM. The
host must set the PCI Memory Mapped I/O Base Ad-
dress register to a value that prevents the Am79C976
controller from claiming any memory cycles not in-
tended for it.
The Am79C976 controller will always read four bytes
for every host Expansion ROM read access. Since this
takes more than 16 PCI clock cycles, the Am79C976
device will assert STOP to force a PCI bus retry. Sub-
sequent accesses will be retried until all four bytes have
been read from the ROM and stored in an internal tem-
porary register. The timing of the access to the ROM
device is determined by the ROMTMG parameter
(CTRL0, bits 11-8).
Note: The Expansion ROM must not be read when the
Am79C976 controller is running (when the RUN bit in
CMD0 is set to 1). Any access to the Expansion ROM
clears the RUN bit and thereby abruptly stops all net-
work and DMA operations.
When the host tries to write to the Expansion ROM, the
Am79C976 controller will claim the cycle. The write op-
8/01/00
P R E L I M I N A R Y
Am79C976
eration will have no effect. Writes to the Expansion
ROM are done through the BCR30 Expansion Bus
Data Port. See the section on the Expansion Bus Inter-
face for more details.
During the boot procedure, the system will try to find an
Expansion ROM. A PCI system assumes that an Ex-
pansion ROM is present when it reads the ROM signa-
ture 55H (byte 0) and AAH (byte 1).
In addition to the normal completion of a transaction,
there are three scenarios in which the Am79C976 con-
troller terminates a slave access for which it is the tar-
get.
If a slave access to the Am79C976 device takes more
than 16 PCI CLK cycles, the Am79C976 device will
generate a PCI disconnect/retry cycle by asserting
STOP and deasserting TRDY while keeping DEVSEL
asserted. This will free up the PCI bus so that it can be
used by other bus masters while the Am79C976 device
is busy. See Figure 55.
The Am79C976 controller cannot service any slave ac-
cess while it is reading the contents of the EEPROM.
Simultaneous access is not allowed in order to avoid
conflicts, since the EEPROM is used to initialize some
of the PCI configuration space locations and user-se-
lected BCRs and CSRs. The EEPROM read operation
will always happen automatically following H_RESET.
(See the H_RESET section for more details.) In addi-
tion, the host can start the read operation by setting the
PREAD bit (BCR19, bit 14). While the EEPROM read
is on-going, the Am79C976 controller will disconnect
any slave access where it is the target by asserting
STOP together with DEVSEL, while driving TRDY high.
STOP will stay asserted until the end of the cycle.
Note: The I/O and memory slave accesses will only be
disconnected if they are enabled by setting the IOEN or
MEMEN bit in the PCI Command register. Without the
enable bit set, the cycles will not be claimed at all.
Since H_RESET clears the IOEN and MEMEN bits for
the automatic EEPROM read after H_RESET, the dis-
connect only applies to configuration cycles.
The Am79C976 device will also generate PCI discon-
nect/retry cycles when it is executing a blocking read
access to an external PHY register.
37