Altera Corporation

49

a8255 Programmable Peripheral Interface Adapter Data Sheet

Note:

(1)

The X indicates “don’t care.”

Notes:

(1)

For example, to reset bit 3 of port C, bit 7 is reset to indicate that the write is in the

port C bit set/reset format. Bits 6 through 4 are “don’t care.” Bits 3 through 1 are

011 to address bit 3, and bit 0 is 0 to indicate a reset operation. The complete data

word is 0XXX0110.

(2)

The X indicates “don’t care.”

Table 3. Control Register Mode Definition Format

Bit

Description

0

Port C (lower) I/O direction:

1 = input

0 = output

1

Port B I/O direction:

1 = input

0 = output

2

Group B mode select:

1 = mode 1

0 = mode 0

3

Port C (upper) I/O direction:

1 = input

0 = output

4

Port A I/O direction:

1 = input

0 = output

6..5

Group A mode select:

00 = mode 0

01 = mode 1

1X = mode 2,

Note (1)

7

1 when writing = mode definition format

Always 1 when reading the control register

Table 4. Port C Bit Set/Reset Format

Note (1)

Bit

Description

0

Bit set/reset:

1 = set

0 = reset

3..1

Bit select address

6..4

XXX,

Note (2)

7

0 when writing = port C bit set/reset format