value that should be stored in the external serial EEPROM is vendor specific. Please see the Section 2.4 “Serial EEPROM Interface”in Chapter 2 for additional information on downloading a value for this register.

Registers: 0x30–0x33

Expansion ROM Base Address

Read/Write

31

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

ERBA[31:0]

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0

0

0

0

0

0

0

 

0

0

 

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

 

0

0

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

ERBA

 

 

 

Expansion ROM Base Address

 

 

 

 

 

 

[31:0]

This four-byte register handles the base address and size information for the expansion ROM. It functions exactly like the Base Address Register Zero (I/O) and One (Memory) registers, except that the encoding of the bits is different. The upper 21 bits correspond to the upper 21 bits of the expansion ROM base address.

The expansion ROM Enable bit, bit 0, is the only bit defined in this register. This bit is used to control whether or not the device accepts accesses to its expansion ROM. When the bit is set, address decoding is enabled, and a device is used with or without an expansion ROM depending on the system configuration. To access the external memory interface, also set the Memory Space bit in the Command register.

The host system detects the size of the external memory by first writing the Expansion ROM Base Address register with all ones and then reading back the register. The LSI53C875A responds with zeros in all don’t care locations. The ones in the remaining bits represent the binary version of the external memory size. For example, to indicate an external memory size of 32 Kbytes, this register, when written with ones and read back, returns ones in the upper 17 bits.

The size of the external memory is set through MAD[3:1]. Please see the section on MAD Bus Programming for the possible size encodings available.

4-12Registers

Page 104
Image 104
LSI 53C875A technical manual Expansion ROM Base Address Read/Write, Erba, Expansion ROM Base Address 310