8255 PIO

24 BIT DIGITAL I/O PIO DETAILS

PROGRAMMING INFORMATION

The 8255 I/O PIO occupies (4) consecutive 8 bit I/O locations starting at BASE+0 and going to BASE+3.

The structure of the addressing is as follows:

• BASE+0 = Port A (read or write)
• BASE+1 = Port B (read or write)
• BASE+2 = Port C (read or write)
• BASE+3 = Control Register (write only)

The following is an overview of each of the three available modes:

MODE 0 (Basic Input/Output):

• (2) Eight bit ports and (2) 4 bit ports
• Any port can be either input or output
• Outputs are latched
• Inputs are not latched
• (16) different Input/Output configurations are possible in this mode

MODE 1 (Strobed Input/Output):

• (2) Groups (Group A and Group B)
• Each group contains one 8 bit data port and (1) 4 bit control/data port
• The 8 bit data port can be either input or output
• Both inputs and outputs are latched
• The 4 bit port is used for control and status of the 8 bit data port

MODE 2 ( Strobed Bi-directional Bus I/O):

• Used in Group A only
• One 8 bit bi-directional bus port (Port A) and one 5 bit control port (Port C)
• Both inputs and outputs are latched
• The 5 bit control port (Port C) is used for control and status for the 8 bit, bi-directional bus port (Port A)

D7 = 1  Designation of mode set flag

D6  D5  Mode selection of Group A
0   0  = mode 0
0   1  = mode 1
1   x  = mode 2

D4  Input/Output selection of Port A
0 = Output
1 = Input

D3  Input/Output selection of high order 4 bits of Port C
0 = Output
1 = Input

D2  Mode selection of Group B
0  = mode 0
1  = mode 1

D1  Input/Output selection of Port B
0 = Output
1 = Input

D0  Input/Output selection of low order 4 bits of Port C
0 = Output
1 = Input

D7 = 0  Bit set/reset flag 0 = Active.

D6 D5 D4 Don't care.

D3 D2 D1 Bit selection
0  0  0  PC0
0  0  1  PC1
0  1  0  PC2
0  1  1  PC3
1  0  0  PC4
1  0  1  PC5
1  1  0  PC6
1  1  1  PC7

D0 Bit set/reset selection
0 = Reset
1 = Set

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