Intel mcs-48 manual 8255A Operational Description, Mode Selection, Single Bit Set/Reset Feature

8255A18255A·5

8255A OPERATIONAL DESCRIPTION

Mode Selection

There are three basic modes of operation that can be select- ed by the system software:

Mode 0 - Basic Input/Output

Mode 1 - Strobed Input/Output

Mode 2 - Bi-Directional Bus

When the reset input goes "high" all ports will be set to the input mode (i.e_, all 24 lines will be in the high im- pedance state)_ AIter the reset is removed the 8255A can remain in the input mode with no additional initialization required_ During the executi.on of the system program any of the other modes may be selected using a single output instruction_ This allows a single 8255A to service a variety of peripheral devices with a simple software maintenance routine_

lIIIt'.

The modes for Port A and Port B can be separately defined, while Port C is divided into two portions as required by the Port A and Port B definitions_ All of the output registers, in- cluding the status fl ip-flops, will be reset whenever the mode is changed_ Modes may be combined so that their functional definition can be "tailored" to almost any I/O structure_ For instance; Group B can be programmed in Mode 0 to monitor simple switch closings or display compu- tational results, Group A could be programmed in Mode 1 to monitor a keyboard or tape reader on an interrupt-driven basis_

ADDRESS BUS

CONTROL BUS

CONTROL WORD

L

I GROUPS \

PORT C (LOWER)

1'"INPUT

•0 '"OUTPUT

PORT B 1'"INPUT 0'"OUTPUT

MODE SELECTION O=MODEO

,,, MODE 1

_I GROUP A

PORT C IUPPERI

1'"INPUT

0= OUTPUT

PORT A

1'"INPUT 0= OUTPUT

MODE SELECTION 00 = MODE 0

01'"MODE 1

lX=MODE2

MODE SET FLAG

1'"ACTIVE

	Figure 4_ Mode Definition Format
MOOED
	The mode definitions and possible mode cOfTlbinations
	may seem confusing at first but after a cursory review of
	the complete device operation a simple, logical 1/0 ap-
	proach will surface_ The design of the 8255A has taken
	into account things such as efficient PC board layout,
	control signal definition vs PC layout and complete
	functional flexibility to support almost any peripheral
	device with no external logic_ Such design represents
	the maximum use of the available pins.
	Single Bit Set/Reset Feature
	Any of the eight bits of Port C can be Set or Reset using a
	single OUTput instruction_ This feature reduces software
Figure 3_ Basic Mode Definitions and Bus Interface	requirements in Control-based applications.

9-20 AFN-00744A-04