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SYM53C825A/825AE Data Manual
Functional Description
PCI Cache Mode
nect to a 32 K B external ROM, use pull-downs on
MAD(3) and MAD(2). If the external memory
interface is not used, then no external resistors are
necessary since there are internal pull-ups on the
MAD bus. T he internal pull-up resistors are dis-
abled when external pull-down resistors are
detected, to reduce current drain.
T he SYM53C825A allows the system to deter-
mine the size of the available external memory
using the Expansion ROM Base Address register
in 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 3.
MAD(0) is the slow ROM pin. When pulled down,
it enables two extra clock cycles of data access time
to allow use of slower memory devices. T he exter-
nal memory interface also supports updates to
flash memory. T he 12 volt power supply for flash
memory, V
PP
, is enabled and disabled with the
GPIO4 pin and the GPIO4 control bit. For more
information on the GPIO4 pin, refer to Chapter 4.
PCI Cache Mode
T he SYM53C825A supports the PCI specification
for an 8-bit Cache Line Size register located in
PCI configuration space. T he Cache Line Size reg-
ister provides the ability to sense and react to non-
aligned addresses corresponding to cache line
boundaries. In conjunction with the Cache Line
Size register, the PCI commands Read Line, Read
Multiple, and Write and Invalidate are each soft-
ware enabled or disabled to allow the user full flex-
ibility in using these commands. For more
information on PCI cache mode operations, refer
to Chapter 3.
Load/Store Instructions
T he SYM53C825A supports the Load/Store
instruction type, which simplifies the movement of
data between memory and the internal chip regis-
ters. It also enables the SYM53C825A to transfer
bytes to addresses relative to the DSA register. For
more information on the Load and Store instruc-
tions, refer to Chapter 6.
3.3 Volt/5 Volt PCI Interface
T he SYM53C825A can attach directly to a 3.3
Volt or a 5 Volt PCI interface, due to separate V
DD
pins for the PCI bus drivers. T his allows the
devices to be used on the universal board recom-
mended by the PCI Special Interest Group.