Intel 80C186XL, 80C188XL user manual 6.4.4Overlapping Chip-Selects, BUS READY R2 Control Bit Wait

CHIP-SELECT UNIT

BUS READY

R2 Control Bit

Wait

Wait State Value (R1:0) State

Counter

READY

Wait

State

Ready

A1137-0A

Figure 6-11. Wait State and Ready Control Functions

The R2 control bit determines whether the bus cycle completes normally (requires bus ready) or unconditionally (ignores bus ready). The R1:0 bits define the number of wait states to insert into the bus cycle. For devices requiring three or fewer wait states, you can set R2 (ignore bus ready) and program R1:0 with the number of required wait states. For devices that may require more than three wait states, you must clear R2 (require bus ready).

A bus cycle with wait states automatically inserted cannot be shortened. A bus cycle that ignores bus ready cannot be lengthened.

6.4.4Overlapping Chip-Selects

The Chip-Select Unit activates all enabled chip-selects programmed to cover the same physical address space. This is true if any portion of the chip-selects’ address ranges overlap (i.e., chip- selects’ ranges do not need to overlap completely to all go active). There are various reasons for overlapping chip-selects. For example, a system might have a need for overlapping a portion of read-only memory with read/write memory or copying data to two devices simultaneously.

If overlapping chip-selects do not have identical wait state and bus ready programming, the Chip- Select Unit uses the following priority scheme:

1.If any MCS chip-select is active, it uses the R2:0 bits in the MPCS register.

2.If the PCS chip-selects overlap LCS, it uses the R2:0 bits in the LMCS register.

3.If the PCS chip-selects overlap UCS, it uses the R2:0 bits in the UMCS register.

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