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SYM53C876/876E Data Manual
Functional Description
Parallel ROM Interface
Parallel ROM Interface
The SYM53C876 supports up to one megabyte
of external memory in binary increments from 16
KB, to allow the use of expansion ROM for add-
in PCI cards. Both functions of the device share
the ROM interface. This interface is designed for
low-speed operations such as downloading
instruction code from ROM; it is not intended for
dynamic activities such as executing instructions.
System requirements include the SYM53C876,
two or three external 8-bit address holding regis-
ters (HCT273 or HCT374), and the appropriate
memory device. The 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 MTD4P05, and several
passive components are also needed. The mem-
ory size and speed is determined by pull-down
resistors on the 8-bit bidirectional memory bus at
power-up. The SYM53C876 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 appropri-
ate conditions.
The external memory interface works with a vari-
ety of ROM sizes and speeds. An example set of
interface drawings is in Appendix C.
The SYM53C876 supports a variety of sizes and
speeds of expansion ROM, using pull-down resis-
tors 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 SYM53C876.
Table 2-5 shows the memory space associated
with the possible values of MAD(3-1). The
MAD(3-1) pins are fully defined in Chapter 3,
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-
nect to a 32 KB external ROM, use pull-downs
on MAD(3) and MAD(2). If the external mem-
ory interface is not used, then no external resis-
tors are necessary since there are internal pull-ups
on the MAD bus. The internal pull-up resistors
are disabled when external pull-down resistors are
detected, to reduce current drain.
The SYM53C876 allows the system to determine
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 4,
Registers.
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. The external memory interface also sup-
ports updates to flash memory.
Table 2-5: Parallel ROM Support
MAD(3-1)
Available Memory Space
000
001
010
011
100
101
110
111
16 KB
32 KB
64 KB
128 KB
256 KB
512 KB
1024 KB
no external memory present