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CS8920A

Crystal LAN ISA Plug-and-Play Ethernet Controller

Cirrus Logic 

CS8920A

the command is a Read, the data in the specified

EEPROM location is written into the EEPROM

Data register. If the command is an Erase or

Erase-All, no data is transferred to or from the

EEPROM Data register. Before issuing any com-

mand, the host must wait for the SIBUSY bit

(Register 16, SelfST, bit 8) to clear. After each

command has been issued, the host must wait

again for SI-BUSY to clear.

Enabling Access to the EEPROM

The Erase/Write Enable command provides pro-

tection from accidental writes to the EEPROM.

The host must write an Erase/Write Enable com-

mand before it attempts to write to or erase any

EEPROM memory location. Once the host has

finished altering the contents of the EEPROM, it

must write an Erase/Write Disable command to

prevent unwanted modification of the EEPROM.

Writing and Erasing the EEPROM

To write data to the EEPROM, the host must

execute the following series of commands:

1. Issue an Erase/Write Enable command.

2. Load the data into the EEPROM Data register.

3. Issue a Write command.

4. Issue an Erase/Write Disable command.

During the Erase command, the CS8920A writes

FFh to the specified EEPROM location. During

the Erase-All command, the CS8920A writes

FFh to all locations.

3.7 Boot PROM Operation

The CS8920A supports an optional Boot PROM

used to store code for remote booting from a

24

CS8920A

CSOUT

(Pin 17)

27C256

20

CE

22 OE

SA(0:14)

74LS245

19

OE

DIR

A1

.

B1

.

. SD(0:7)

.

.

A8

.

B8

ISA

BUS

Figure 3.3. Boot PROM Connection Diagram

network server. This is typically done for a disk-

less workstation.

Accessing the Boot PROM

The CS8920A provides address decoding cir-

cuitry to generate a chip select for a Boot

PROM. When the address on the ISA bus match

the address loaded into the Boot PROM base ad-

dress register and qualified by Boot PROM

address mask register. The CS8920A generates a

chipselect signal for the Boot PROM.

Configuring the CS8920A for Boot PROM

Operation

Figure 3.3 show how the CS8920A should be

connected to the Boot PROM and ’245 driver.

To configure the CS8920A’s internal registers for

Boot PROM operation, the Boot PROM Base

Address must be loaded into the Boot PROM

Base Address register (PacketPage base + 0340h)

and the Boot PROM Address Mask must be

loaded into the BootPROM Address Mask regis-

ter (PacketPage base + 0343h). The Boot PROM

Base Address provides the starting location in

host memory where the Boot PROM is mapped.

The Boot PROM Address Mask indicates the

size of the attached Boot PROM and is limited

to 4-Kbyte increments. The lower 12 bits of the

Address Mask are ignored and should be 000h.

DS238PP2

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