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I/O Space
4-25
Memory and I/O Spaces
4.6.1 Accessing I/O Space
All I/O words (external I/O ports and on-chip I/O registers) are accessed with
the IN and OUT instructions. Accesses to external parallel I/O ports are multi-
plexed over the same address and data buses for program and data-memory
accesses. These accesses are distinguished from external program and data-
memory accesses by IS
going low. The data bus is 16 bits wide; however, if
you use 8-bit peripherals, you can use either the higher or lower eight lines of
the data bus to suit a particular application.
You can use RD
with chip-select logic to generate an output-enable signal for
an external peripheral. You can also use the WE
signal with chip-select logic
to generate a write-enable signal for an external peripheral. As an example of
interfacing to external I/O space, Figure 4–13 shows interface circuitry for
eight input bits and eight output bits. Note that the decode section is simplified
if fewer I/O ports are used.