System Integration Module
5-8
MC68307 USER’S MANUAL
MOTOROLA
5.1.2.3 8051-COMPATIBLE BUS CHIP SELECT. Chip select 3 (CS3 signal) can be used
to define the addressing range of the 8051-compatible bus mode, when this mode is
enabled in the SCR E8051 bit. Otherwise (if the 8051-compatible bus mode is not used) chip
select 3 is available for any general purpose memory or peripheral. In this case, the 8051-
compatible bus read and write strobes (RD and WR), and the address latch enable (ALE)
signal are always negated. This bus always uses an 8-bit data-bus width, and so the
BUSW3 bit in the SCR should be set along with the E8051 bit.
5.1.2.4 GLOBAL CHIP SELECT OPERATION (RESET DEFAULTS). Chip
select
0
is
initialized from cold reset to assert in response to any address in the first 8K bytes of memory
space, in order to ensure a chip select to the boot ROM or EPROM, to fetch the reset vector
and execute the initialization code, which should set up the module base address and the
four chip select ranges early on in that initialization sequence.
The data bus port size for CS0 on reset, and hence the data width of the boot ROM device,
are programmed by placing logic 0 or 1 on the BUSW pin during reset, for 8-bit and 16-bit
wide data bus respectively.
The other 3 chip selects are initialized to be invalid, and so do not assert until they are
programmed.
5.1.2.5 OVERLAP IN CHIP SELECT RANGES. The user should not normally program
more than one chip select line to the same area. If this accidentally occurs, only one chip
select line is driven because of internal line priorities. CS0 has the highest priority, and CS3
the lowest. The address compare logic sets the address decode conflict (ADC) status bit in
the SCR, and also generates a bus error (or BERR is asserted) if the address decode
conflict enable (ADCE) bit was set by the user in the SCR.
BERR is never asserted on write accesses to the chip select registers.
If one chip select is programmed to be read-only, and another is programmed to be write-
only, then there is no overlap conflict between these two chip selects, and the address
decode conflict (ADC) status bit in the SCR is not set.
When the CPU attempts to write to a read-only location, as programmed by the user when
setting up the chip selects, the chip select logic sets the write protect violation (WPV) bit in
the SCR, and will also generate BERR if the write protect violation enable (WPVE) bit is set
in the SCR. The CSx line is not asserted.
NOTE
The chip select logic is reset only on cold reset (assertion of
RESET and HALT, or RSTIN). The chip select (CSx) lines are
never asserted on accesses to the MBAR and SCR locations.
Thus, it is very convenient to use CSx lines to select external
ROM/RAM that overlaps or encloses the MBAR and SCR