Department of Computer Scien

ce and Information Engineering

National Cheng Kung University, TAIWAN

HANEL

PROGRAMMING THE 8255Connecting 8031/51 to

(^8255) (cont’)

¾or, see the following, also using EQU:

CONTRBYT EQU 90H

;(PA=IN, PB=OUT, PC=OUT)

BAS8255P EQU 1000H

;base address for 8255

MOV A,#CONTRBYTMOV DPTR,#BAS8255P+3

;load c port addr

MOVX @DPTR,A ;issue control word MOV DPTR,#BAS8255P+3 ;PA address...

‰Example 15-2 and 15-3

¾use the DPTR register since the base address assigned to 8255 was 16-bit¾if it was 8-bit, we can use “MOVX A,@R0

”and

“MOVX @R0,A

”

‰Example 15-4

¾use a logic gate to do address decoding

‰Example 15-5

¾use a 74LS138 for multiple 8255s