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SYM53C876/876E Data Manual
Functional Description
Serial EEPROM Interface
Mode B Operation
A 4.7K pulldown on MAD6, no pulldown on
MAD7.
In this mode, GPIO0 and GPIO1 are each
defined as either the serial data signal (SDA) or
the serial clock signal (SCL), since both pins are
controlled through software.
No data is automatically loaded into chip registers
at power-up or hard reset. The Subsystem ID reg-
ister and Subsystem Vendor ID register are read/
write, in violation of the PCI specification, with a
default value of all zero’s.
Mode C Operation
A 4.7K pulldown on MAD6, and a 4.7K pull-
down on MAD7.
In this mode, GPIO1 is the serial data signal
(SDA) and GPIO0 is the serial clock signal
(SCL). Certain data in the serial EEPROM is
automatically loaded into chip registers at power-
up or hard reset.
The format of the serial EEPROM data is defined
in Table 2-7. If the EEPROM is not present, or
the checksum fails, the Subsystem ID (SID)and
Subsystem Vendor ID (SVID) registers read back
all zeros. At power-up or hard reset, only five
bytes are loaded into the chip from locations FBh
through FFh.
The SID and SVID registers are read only, in
accordance with the PCI specification, with a
default value of all zeros.
Before implementing Mode C, contact Symbios
Logic for additional information.
Table 2-7: Mode C Serial EEPROM data format
Byte
Name
Description
00h-
FAh
FBh
UD0
User Data
SVID(0)
Subsystem Vendor ID, LSB.
This byte is loaded into the
least significant byte of the
Subsystem Vendor ID register
in the appropriate PCI config-
uration space at chip power-up
or hard reset.
Subsystem Vendor ID, MSB.
This byte is loaded into the
most significant byte of the
Subsystem Vendor ID register
in the appropriate PCI config-
uration spaces at chip power-
up or hard reset.
Subsystem ID, LSB. This byte
is loaded into the least signifi-
cant byte of the Subsystem ID
register in the appropriate PCI
configuration space at chip
power-up or hard reset.
Subsystem ID, MSB. This
byte is loaded into the most
significant byte of the Sub-
system ID register in the
appropriate PCI configura-
tion space at chip power-up or
hard reset.
Checksum. This 8-bit check-
sum is formed by adding,
bytewise, each byte contained
in locations 00h-03h to the
seed value 55h, and then tak-
ing the 2’s compliment of the
result.
User Data.
FCh
SVID(1)
FDh
SID(0)
FEh
SID(1)
FFh
CKSUM
100h-
EOM
UD