80C186EC/80C188EC Microprocessor User's Manual

BUS INTERFACE UNIT
(X + 1)
(X)
A19:1 D15:8 BHE
D7:0 A0
(Low)
(Low)
A1107-0A
Figure 3-3. 16-Bit Data Bus Even Word Transfers
During a byte read operation, the BIU floats the entire 16-bit data bus, even though the transfer
occurs on only one half of the bus. This action simplifies the decoding requirements for read-only
devices (e.g., ROM, EPROM, Flash). During the byte read, an external device can drive both
halves of the bus, and the BIU automatically accesses the correct half. During the byte write operation,
the BIU drives both halves of the bus. Information on the half of the bus not involved in
the transfer is indeterminate. This action requires that the appropriate bank (defined by BHE or
A0 high) be disabled to prevent destroying data.
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