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3.9 MAD Bus Programming
3-14
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
4.7 k
resistor between the appropriate MAD[x] pin and V
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
EEPROM Interface,"
Vendor ID
registers in
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
001
010
011
100
101
110
111
no external memory present
Signal Descriptions
in
Chapter 2
and
Subsystem ID
Chapter 4
for additional information.
16 Kbyte
32 Kbyte
64 Kbyte
128 Kbyte
256 Kbyte
512 Kbyte
1024 Kbyte
. The
SS
Section 2.4, "Serial
and
Subsystem
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