AM79C971VCW datasheet(95/265 Pages) AMD

95 / 265 page

Am79C971

If the user wishes to modify any of the configuration bits

that are contained in the EEPROM, then the seven

command, data and status bits of BCR19 can be used

to write to the EEPROM. After writing to the EEPROM,

the host should set the PREAD bit of BCR19. This

action forces an Am79C971 controller reread of the

EEPROM so that the new EEPROM contents will be

loaded into the EEPROM-programmable registers on

board the Am79C971 controller. (The EEPROM-pro-

grammable registers may also be reprogrammed di-

rectly, but only information that is stored in the

EEPROM will be preserved at system power-down.)

When the PREAD bit of BCR19 is set, it will cause the

Am79C971 controller to ignore further accesses to the

Am79C971 configuration space, the Expansion ROM,

or any I/O resource until the completion of the EE-

PROM read operation. The Am79C971 controller will

terminate these access attempts with the assertion of

DEVSEL and STOP while TRDY is not asserted, sig-

naling to the initiator to disconnect and retry the access

at a later time.

EEPROM Auto-Detection

The Am79C971 controller uses the EESK/LED1/SFBD

pin to determine if an EEPROM is present in the sys-

tem. At the rising edge of CLK during the last clock dur-

ing which RST is asserted, the Am79C971 controller

will sample the value of the EESK/LED1/SFBD pin. If

the sampled value is a 1, then the Am79C971 controller

assumes that an EEPROM is present, and the EE-

PROM read operation begins shortly after the RST pin

is deasserted. If the sampled value of EESK/LED1/

SFBD is a 0, the Am79C971 controller assumes that an

external pulldown device is holding the EESK/LED1/

SFBD pin low, indicating that there is no EEPROM in

the system. Note that if the designer creates a system

that contains an LED circuit on the EESK/LED1/SFBD

pin, but has no EEPROM present, then the EEPROM

auto-detection function will incorrectly conclude that an

EEPROM is present in the system. However, this will

not pose a problem for the Am79C971 controller, since

the checksum verification will fail.

Direct Access to the Interface

The user may directly access the port through the

EEPROM register, BCR19. This register contains bits

that can be used to control the interface pins. By per-

forming an appropriate sequence of accesses to

BCR19, the user can effectively write to and read from

the EEPROM. This feature may be used by a system

configuration utility to program hardware configuration

information into the EEPROM.

EEPROM-Programmable Registers

The following registers contain configuration informa-

tion that will be programmed automatically during the

EEPROM read operation:

s I/O offsets 0h-Fh Address PROM locations

s BCR2

Miscellaneous Configuration

s BCR4

LED0 Status

s BCR5

LED1 Status

s BCR6

LED2 Status

s BCR7

LED3 Status

s BCR9

Full-Duplex Control

s BCR18

Burst and Bus Control

s BCR22

PCI Latency

s BCR23

PCI Subsystem Vendor ID

s BCR24

PCI Subsystem ID

s BCR25

SRAM Size

s BCR26

SRAM Boundary

s BCR27

SRAM Interface Control

s BCR32

MII Control and Status

s BCR33

MII Address

s BCR35

PCI Vendor ID

If PREAD (BCR19, bit 14) and PVALID (BCR19, bit 15)

are cleared to 0, then the EEPROM read has experi-

enced a failure and the contents of the EEPROM pro-

gram mable BCR register will be set to default

H_RESET values. The content of the Address PROM

locations, however, will not be cleared.

Note that accesses to the Address PROM I/O locations

do not directly access the Address EEPROM itself. In-

stead, these accesses are routed to a set of shadow

registers on board the Am79C971 controller that are

loaded with a copy of the EEPROM contents during the

automatic read operation that immediately follows the

H_RESET operation.

EEPROM MAP

The automatic EEPROM read operation will access 32

words (i.e., 64 bytes) of the EEPROM. The format of

the EEPROM contents is shown in Table 14, beginning

with the byte that resides at the lowest EEPROM ad-

dress.

Note that the first bit out of any word location in the EE-

PROM is treated as the MSB of the register being pro-

grammed. For example, the first bit out of EEPROM

word location 09h will be written into BCR4, bit 15; the

second bit out of EEPROM word location 09h will be

written into BCR4, bit 14, etc.

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