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Chip Select Unit
5-3
When overlapping an additional chip select with either the LCS or UCS chip selects, it must
be noted that setting the Disable Address (DA) bit in the LMCS or UMCS Register will
disable the address from being driven on the AD bus for all accesses for which the
associated chip select is asserted, including any accesses for which multiple chip selects
assert.
The MCS
and PCS chip select pins can be configured as either chip selects (normal
function) or as PIO inputs or outputs. It should be noted, however, that the ready and wait
state generation logic for these chip selects is in effect regardless of their configurations
as chip selects or PIOs. This means that if these chip selects are enabled (by a write to the
MMCS and MPCS registers for the MCS
chip selects, or by a write to the PACS and MPCS
registers for the PCS
chip selects), the ready and wait state programming for these signals
must agree with the programming for any other chip selects with which their assertion would
overlap if they were configured as chip selects.
Although the PCS
4 signal is not available on an external pin, the ready and wait state logic
for this signal still exists internal to the part. For this reason, the PCS
4 address space must
follow the rules for overlapping chip selects. The ready and wait-state logic for PCS
6–PCS5
is disabled when these signals are configured as address bits A2–A1.
Failure to configure overlapping chip selects with the same ready and wait state
requirements may cause the processor to hang with the appearance of waiting for a ready
signal. This behavior may occur even in a system in which ready is always asserted (ARDY
or SRDY tied High).
Configuring PCS in I/O space with LCS or any other chip select configured for memory
address 0 is not considered overlapping of the chip selects. Overlapping chip selects refers
to configurations where more than one chip select asserts for the same physical address.
5.5 CHIP SELECT REGISTERS
The following sections describe the chip select registers.