3.9 MAD Bus Programming

The MAD[7:0] pins, in addition to serving as the address/data bus for the local memory interface, also are used to program power-up options for the chip. A particular option is programmed allowing the internal pull-down current sink to pull the pin LOW at reset or by connecting a

4.7kresistor between the appropriate MAD[x] pin and VSS. The pull-down resistors require that HC or HCT external components are

used for the memory interface. The MAD[7:0] pins are sensed by internal circuitry three PCI clock cycles after RST/ is deasserted.

MAD[7] Serial EEPROM programmable option. When allowed to be pulled LOW by the internal pull-down current sink, the automatic data download is enabled. When pulled HIGH by an external resistor, the automatic data download is disabled. Please see Section 2.4, “Serial EEPROM Interface,” in Chapter 2 and Subsystem ID and Subsystem Vendor ID registers in Chapter 4 for additional information.

MAD[6:4] Reserved and may be left floating.

The MAD[3:1] pins are used to set the size of the external expansion ROM device attached. Encoding for these pins are listed in

Table 3.15 (“0” indicates a pull-down resistor is attached, “1” indicates a pull-up resistor is attached).

Table 3.15 Decode of MAD Pins

MAD[3:1]

Available Memory Space

 

 

000

16 Kbyte

 

 

001

32 Kbyte

 

 

010

64 Kbyte

 

 

011

128 Kbyte

 

 

100

256 Kbyte

 

 

101

512 Kbyte

 

 

110

1024 Kbyte

 

 

111

no external memory present

 

 

3-14

Signal Descriptions

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LSI 53C875A technical manual MAD Bus Programming, Decode of MAD Pins