
SYM53C895 Interfaces
2-13
example, to connect to a 32 Kbytes external ROM, use pull-downs on
MAD(3) and MAD(2) and a pull-up on MAD(1).
Note:
The SYM53C875 contains internal pull-ups on the MAD
bus. The SYM53C895 requires external resistors to pull up
the MAD bus to VDD.
The SYM53C895 allows the system to determine the size of the available
external memory using the Expansion ROM Base Address register in the
PCI conguration space. For more information on how this works, refer
to the PCI specication or the Expansion ROM Base Address register
MAD(0) is the slow ROM pin. When pulled down, it enables two extra
clock cycles of data access time, which allows use of slower memory
devices.
The external memory interface also supports updates to ash memory.
The 12-volt power supply for ash memory, VPP, is enabled and disabled
with the GPIO4 pin and the GPIO4 control bit. For more information on
2.6.2 Serial EEPROM Interface
The SYM53C895 implements an interface that allows attachment of a
serial EEPROM device to the GPIO0 and GPIO1 pins. Four different
modes of operation are possible; each one relates to different values for
the serial EEPROM interface, the Subsystem ID Register, and the
Subsystem Vendor ID register. The modes are programmable through
the MAD6 and MAD7 pins, which are sampled at power-up or hard reset.
2.6.2.1 Mode A: 4.7 K
Pull-ups on MAD6 and MAD7
In this mode, GPIO0 is the serial data signal (SDA) and GPIO1 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 dened in
Table 2.2. If the
EEPROM is not present, or the checksum fails, the Subsystem ID and
Subsystem Vendor ID registers read back all zeroes. At power-up or hard
reset, only ve bytes are loaded into the chip from locations 0x00 through
0x04.