Zilog Z08470 - Manual (Page 209)

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UM008101-0601 Parallel Input/Output
Selecting An Operating Mode
PIO Port A allows operation in four modes: Mode 0 (output mode), Mode 1
(input mode), Mode 2 (bidirectional mode), and Mode 3 (control mode).
The mode numbers are assigned for mnemonic significance: 0 = Out,
1 = In, 2 = Bidirectional. Port B cannot operate in Mode 2.
The mode of operation must be established by writing a control word to the
PIO in the following format:
Listed in Table 1, Bits D7 and D6 form the binary code that designate mode
of operation:
Bits D5 and D4 are ignored. Bits D3-D0 must be set to 1111 to indicate
Set Mode.
Selecting Mode 0 enables any data written to the port output register by
the CPU to be enabled onto the port data bus. The output register contents
may be changed by writing a new data word to the port. Also, the current
contents of the output register may be read back to the Z80 CPU by
executing an input instruction.
When Mode 0 is active, a data write from the CPU causes the port’s
Ready handshake line to go High, thereby notifying the peripheral that
Table 1. PIO Mode Selection
D7 D6 Mode
0 0 0 (output)
0 1 1 (input)
1 0 2 (bidirectional)
1 1 3 (control)
Mode Word
M1 M0 X X
Signifies Mode Word to be Set
D7 D6 D5 D4 D3 D0 D2 D1
1 1 1 1