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Functional Description
External Memory Interface
SYM53C825A/825AE Data Manual
2-3
contents, use the No Flush option for all Store
operations that do not modify code within the
next 8 dwords.
3. On every write to the DSP.
4. On all Transfer Control instructions when the
transfer conditions are met. T his is necessary
because the next instruction to be executed is
not the sequential next instruction in the
prefetch unit.
5. When the Pre-Fetch Flush bit (DCNT L bit 6)
T he bit is self-clearing.
Op Code Fetch
Burst Capability
Setting the Burst Op Code Fetch Enable bit in the
DMODE register (38h) causes the SYM53C825A
to burst in the first two longwords of all instruction
fetches. If the instruction is a memory-to-memory
move, the third longword will be accessed in a sep-
arate ownership. If the instruction is an indirect
type, the additional longword will be accessed in a
subsequent bus ownership. If the instruction is a
table indirect Block Move, the SYM53C825A will
use two accesses to obtain the four longwords
required, in two bursts of two longwords each.
Note: T his feature is only useful if pre-fetching is
disabled.
External Memory Interface
T he SYM53C825A supports up to one megabyte
of external memory in binary increments from 16
K B, to allow the use of expansion ROM for add-in
PCI cards. T he device also supports flash ROM
updates through the add-in interface and the
GPIO4 pin (used to control V
PP
, the power supply
for programming external memory). T his interface
is designed for low-speed operations such as down-
loading instruction code from ROM; it is not
intended for dynamic activities such as executing
instructions.
System requirements include the SYM53C825A,
two or three external 8-bit address holding regis-
ters (HCT 273 or HCT 374), and the appropriate
memory device. T he 4.7 K
pull-down resistors
on the MAD bus require HC or HCT external
components to be used. If in-system flash ROM
updates are required, a 7406 (high voltage open
collector inverter), an MT D4P05, and several pas-
sive components are also needed. T he memory size
and speed is determined by pull-down resistors on
the 8-bit bidirectional memory bus at power up.
T he SYM53C825A senses this bus shortly after
the release of the Reset signal and configures the
ROM Base Address register and the memory cycle
state machines for the appropriate conditions.
T he external memory interface works with a vari-
ety of ROM sizes and speeds. An example set of
interface drawings is in Appendix C.
T he SYM53C825A supports a variety of sizes and
speeds of expansion ROM, using pull-down resis-
tors on the MAD(3-0) pins. T he encoding of pins
MAD(3-1) allows the user to define how much
external memory is available to the
SYM53C825A. Table 2-1 shows the memory
space associated with the possible values of
MAD(3-1). T he MAD(3-1) pins are fully defined
in Chapter 4, “Signal Descriptions.”
To use one of the configurations mentioned above
in a host adapter board design, put 4.7 K
pull-
down resistors on the MAD pins corresponding to
the available memory space. For example, to con-
Table 2-1: E xternal Memory Support
MAD(3-1)
Available Memory Space
000
001
010
011
100
101
110
111
16 K B
32 K B
64 K B
128 K B
256 K B
512 K B
1024 K B
no external memory present