Department of Computer Scien

ce and Information Engineering

National Cheng Kung University, TAIWAN

HANEL

PROGRAMMING THE 82558255 Features

(cont’)

‰A0, A1, and CS (chip select)

¾CS is active-low¾While CS selects the entire chip, it is A0 and A1 that select specific ports¾These 3 pins are used to access port A, B, C, or the control register

(^8255) A
PA3PA2PA1PA0-RD-CSGNDA1A0PC7PC6PC5PC4PC0PC1PC2PC3PB0PB1PB2
PA4PA5PA6PA7-WRRESETD0D1D2D3D4D5D6D7VccPB7PB6PB5PB4PB3
(^1234567891011121314151617181920)
4039383736353433323130292827262524232221
8255 is not selected
X
X
1
Control register
1
1
0
Port C
0
1
0
Port B
1
0
0
Port A
0
0
0
Selection
A0
A1
8255 Port SelectionCS