LSI 53C875A technical manual Parallel ROM Support, MAD31 Available Memory Space

The LSI53C875A supports a variety of sizes and speeds of expansion ROM, using pull-down resistors on the MAD[3:0] pins. The encoding of pins MAD[3:1] allows the user to define how much external memory is available to the LSI53C875A. Table 2.6 shows the memory space associated with the possible values of MAD[3:1]. The MAD[3:1] pins are fully described in Chapter 3, “Signal Descriptions.”

Table 2.6	Parallel ROM Support
MAD[3:1]		Available Memory Space
000		16 Kbytes
001		32 Kbytes
010		64 Kbytes
011		128 Kbytes
100		256 Kbytes
101		512 Kbytes
110		1024 Kbytes
111		no external memory present

To use one of the configurations mentioned above in a host adapter board design, put 4.7 kΩ pull-up resistors on the MAD pins corresponding to the available memory space. For example, to connect to a 64 Kbyte external ROM, use a pull-up on MAD2. If the external memory interface is not used, MAD[3:1] should be pulled HIGH.

Note: There are internal pull-downs on all of the MAD bus signals.

The LSI53C875A allows the system to determine the size of the available external memory using the Expansion ROM Base Address register in the PCI configuration space. For more information on how this works, refer to the PCI specification or the Expansion ROM Base Address register description in Chapter 4, “Registers.”

MAD0 is the slow ROM pin. When pulled up, it enables two extra clock cycles of data access time to allow use of slower memory devices. The external memory interface also supports updates to flash memory.

Parallel ROM Interface 2-49