Configurable Options
When using on-board battery backup, the ZT 8825 uses a voltage
comparator to detect when the Vcc supply is less than 4.5 V. When
this is true, the ZT 8825 prevents any access to the SRAMs. This
comparator can also drive the DCLOW* signal (pin 6, low true) to
disable other boards to protect the RAM and/or EEPROM data.
Installing Jumper W22 will assert DCLOW*. PBRESET* can be
asserted by installing Jumper W23.
I/O BASE ADDRESS SELECTION
Before choosing the base addresses for the four registers, you must
decide on 8-bit or 16-bit addressing. The concept is the same in both
cases, except for the number of bits used to define the 16-bit base
address.
You must choose a base address that does not conflict with any other
I/O board that exists on the STD bus. Any overlap causes conflict on
the data bus; it can produce invalid data and cause the data bus driver
ICs to fail. We suggest you draw a complete I/O map of the system
showing starting addresses and ranges for each board in the system
before selecting the base address for the ZT 8825. This is especially
true if 8-bit and 16-bit address I/O boards will exist on the same
backplane. IOEXP is often used to resolve these difficulties. This
signal is always used in determining the validity of an I/O access to
this board. You must choose whether IOEXP must be HIGH (Jumper
W15 OUT) or LOW (Jumper W15 IN) to allow access to the board.
5-10
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