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System Integration Block (SIB)
address match exists within its address space and, therefore, whether to assert the chip-
select line.
111 = Not supported; reserved. Chip select will not assert if this value is chosen.
110 = Value may be used.
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000 = Value may be used.
After system reset, the FC field in BR3–BR0 defaults to supervisor program space (FC =
110) to select a ROM device containing the reset vector. Because of the priority mechanism
and the EN bit, only the CS0 line is active after a system reset.
The FC bits can be masked and ignored by the chip-select logic
using CFC in the OR.
Bits 12–2—Base Address
These bits are used to set the starting address of a particular address space. The address
compare logic uses only A23–A13 to cause an address match within its block size. The
base address should be located on a block boundary. For example, if the block size is 64k
bytes, then the base address should be a multiple of 64k.
After system reset, the base address defaults to zero to select a ROM device on which
the reset vector resides. All base address values default to zero on system reset, but, be-
cause of the priority mechanism, only CS0 will be active.
All address bits can be masked and ignored by the chip-select
logic through the base address mask in the OR.
RW—Read/Write
0 = The chip-select line is asserted for read operations only.
1 = The chip-select line is asserted for write operations only.
After system reset, this bit defaults to zero (read-only operation).
This bit can be masked and ignored by the read-write compare
logic, as determined by MRW in the OR. The line is then assert-
ed for both read and write cycles.
On write protect violation cycles (RW = 0 and MRW = 1), BERR will be generated if WPVE
is set, and WPV will be set.
If the write protect mechanism is used by an external master, the R/W low to AS asserted
timing should be 16 ns minimum.
3-46
NOTE
NOTE
NOTE
MC68302 USER'S MANUAL
MOTOROLA
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