AMD Am188TMER, Am186TMER user manual Chip Select Registers

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 PCS4 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 PCS4 address space must follow the rules for overlapping chip selects. The ready and wait-state logic for PCS6–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.

5.5CHIP SELECT REGISTERS

The following sections describe the chip select registers.

Chip Select Unit 5-3