AM79C974 datasheet(60/153 Pages) AMD

60 / 153 page

AMD

P R E L I M I N A R Y

Am79C974

H_RESET function.

EEPROM programming of the DWIO mode bit of

BCR18.

Automatic determination of DWIO mode due to

DWORD (double-word) I/O write access to offset

10h.

The Ethernet controller I/O mode setting will default to

WIO after H_RESET (i.e. DWIO = 0).

Following the H_RESET operation, a PREAD operation

of the EEPROM will be executed. If the EEPROM is pro-

grammed with a ONE in the DWIO bit position and the

EEPROM checksum is correct, then the EEPROM read

will result in the setting of the DWIO mode to a ONE.

If the EEPROM programming was absent, failed, or con-

tained a ZERO in the DWIO bit position, then the soft-

ware may invoke DWIO mode by performing a Double

Word write access to the I/O location at offset 10h

(RDP). Note that even though the I/O resource mapping

changes when the I/O mode setting changes, the RDP

location offset is the same for both modes.

1-, 2- and 3-byte accesses to Ethernet controller I/O re-

sources are not allowed during DWIO mode.

The mapping of the Ethernet controller resources into

the 32-byte I/O space varies depending upon the setting

of the DWIO bit of BCR10. Depending upon the setting

of this variable, the 32-byte I/O space will be either Word

I/O mapped (WIO) or Double Word I/O mapped (DWIO).

A DWIO setting of 0 produces Word I/O mode, while a

DWIO setting of 1 produces Double Word I/O mapping.

DWIO is automatically programmed as active when the

system attempts a DWORD write access to offset 10h of

the Ethernet controller I/O space. As long as no

DWORD write access to offset 10h of the Ethernet con-

troller I/O space is performed, the Ethernet controller

will use the value of DWIO that was programmed from

the EEPROM read operation. If no EEPROM is used in

the system, then the power up reset value of DWIO will

be ZERO, and this value will be maintained until a

DWORD access is performed to Ethernet controller I/O

space.

Therefore, if DWIO mode is desired, it is imperative that

the first access to the Ethernet controller be a DWORD

write access to offset 10h, unless the EEPROM will be

used to program the DWIO bit.

Alternatively, if DWIO mode is not desired, then it is im-

perative that the software never executes a DWORD

write access to offset 10h of the Ethernet controller I/O

space, and the EEPROM programming of the DWIO bit

must be ZERO.

Once the DWIO bit has been set to a ONE, only a hard-

ware H_RESET or a new read of the EEPROM can re-

set it to a ZERO.

The DWIO mode setting is unaffected by the S_RESET

or setting the STOP bit.

WIO I/O Resource Map

When the Ethernet controller I/O space is mapped as

Word I/O, then the resources that are allotted to the

Ethernet controller occur on word boundaries that are

offset from the Ethernet controller I/O base address as

shown in the table below:

No. of

Offset

Bytes

APROM

10h

RDP

12h

RAP (shared by RDP and BDP)

14h

Reset Register

16h

BDP

18h

Vendor Specific Word

1Ah

Reserved

1Ch

Reserved

1Eh

Reserved

When Ethernet controller I/O space is Word mapped, all

I/O resources fall on word boundaries and all I/O re-

sources are word quantities. However, while in Word I/O

mode, APROM locations may also be accessed as indi-

vidual bytes either on odd or even byte addresses.

Attempts to write to any Ethernet controller I/O re-

sources

(except to offset 10h, RDP) as 32 bit quantities

while in Word I/O mode are illegal and may cause unex-

pected reprogramming of the Ethernet controller control

registers. Attempts to

read from any Ethernet controller

I/O resources as 32-bit quantities while in Word I/O

mode are illegal and will yield undefined values.

An attempt to write to offset 10h (RDP) as a 32 bit quan-

tity while in Word I/O mode will cause the Ethernet con-

troller to exit WIO mode and immediately thereafter, to

enter DWIO mode.

Word accesses to non word address boundaries are not

allowed while in WIO mode. (A write access may cause

unexpected reprogramming of the Ethernet controller

control registers. A read access will yield undefined

values.)

AM79C974 Datasheet(PDF) 60 Page - Advanced Micro Devices

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