System Integration Module
5-26
MC68307 USER’S MANUAL
MOTOROLA
E8051—Enable 8051-Compatible Bus
This bit enables or disables the expansion of chip select 3 into an 8051-compatible bus
interface, with ALE, RD and WR pins indicating the timing of an 8-bit multiplexed lower
0 = The CS3 output pin functions as a generic M68000-bus chip select.
1 = The CS3 output pin functions as a chip select for the 8051-compatible bus. The ad-
dress latch enable output and RD/WR output strobes are active for this bus cycle.
Each bus cycle has timing which is compatible with 8051 memory and peripherals,
rather than M68000 ones. The lower 8 address lines function as a multiplexed ad-
dress/data bus. The BUSW3 bit should be programmed for 8-bit bus width (value
zero), as the 8051-compatible bus necessitates an 8-bit data-bus width.
BUSW0—Bus Width for Chip-Select 0
This bit defines the data bus width (port size) for the memory device selected by CS0,
which is usually the boot ROM device. BUSW0 controls the bus sizing logic whenever
CS0 asserts on an address match.
0 = The bus width for CS0-controlled memory accesses is 8-bits.
1 = The bus width for CS0-controlled memory accesses is 16-bits.
After cold reset, this bit takes on the logic value which was present on the BUSW external
pin during the reset, thus allowing the boot ROM data width to be selected.
BUSW1—Bus Width for Chip-Select 1
This bit defines the data bus width (port size) for the memory device selected by CS1.
BUSW1 controls the bus sizing logic whenever CS1 asserts on an address match.
0 = The bus width for CS1-controlled memory accesses is 8-bits.
1 = The bus width for CS1-controlled memory accesses is 16-bits.
After cold reset, this bit defaults to one, so the default width for CS1-controlled memory
accesses is 16-bits.
BUSW2—Bus Width for Chip-Select 2
This bit defines the data bus width (port size) for the memory device selected by CS2 and
also the four peripheral chip select outputs (CS2A, CS2B, CS2C, CS2D) if they are en-
abled by the EPCS bit. BUSW2 controls the bus sizing logic whenever CS2 asserts on an
address match.
0 = The bus width for CS2-controlled memory/peripheral accesses is 8-bits.
1 = The bus width for CS2-controlled memory/peripheral accesses is 16-bits.
After cold reset, this bit defaults to one, so the default width for CS2-controlled memory
accesses is 16-bits.
BUSW3—Bus Width for Chip-Select 3
This bit defines the data bus width (port size) for the M68000 memory device selected by
CS3. BUSW3 controls the bus sizing logic whenever CS3 asserts on an address match.