8255 I/O Interfacing

CSC-167 · Semester II · Microprocessor

intermediate35 minAvailable

8255 Programmable I/O

Build the control word and see how each port is configured.

Group A mode

Port A

Port B

Port C lower

Control word

0x82

10000010

Bit 7 = 1 (I/O mode) · Bits 6-5 = Group A mode 0 · Bit 4 = Port A out

Bit 3 = Port C upper out · Bit 2 = Group B mode 0 · Bit 1 = Port B in · Bit 0 = Port C lower out

Software writes the control word to port 0x43 to configure the chip at runtime.

Lab brief

Objective

Program a simulated 8255 in mode 0, configure port directions with a control word, and move data through ports A and B.

Theory & concepts

The 8255 gives the 8085 three 8-bit ports whose directions are set by a single control word. Writing the control word to the control register (port 03H in a 00H-03H decode) configures everything; after that, IN and OUT transfer data through the ports.

Rendering diagram…

8085 talking to the 8255 through its ports

Port	Address (decode 00H-03H)	Direction in the example
Port A	00H	Input (reads switches)
Port B	01H	Output (drives LEDs)
Port C	02H	Output (low nibble), input (high nibble)
Control register	03H	Write-only (control word)

; 8255 at ports 00H-03H, mode 0:
;   A = input, B = output, C low = output, C high = input.
; Control word: 1 00 1 1 0 0 0 = 98H

        MVI A, 98H
        OUT 03H        ; write the control word

READ:   IN 00H         ; A = switch pattern from port A
        OUT 01H        ; show it on the LEDs at port B
        JMP READ       ; continuous echo

Control word vs data

The control register is write-only in mode-set form. A control word has bit 7 = 1. If you accidentally write data to port 03H, you reconfigure the 8255 instead of outputting data.

Prerequisites

Lesson: 8255 programmable peripheral interface
Lesson: 8255 modes and control words
Lesson: Basic I/O operations: memory-mapped and I/O-mapped

Instructions

1Place the 8255 at ports 00H-03H and attach 8 switches to port A and 8 LEDs to port B.
2Write and run the initialization: MVI A, 98H followed by OUT 03H.
3Change the switch pattern and confirm the LEDs mirror port A exactly.
4Reconfigure with control word 90H (A input, B output, C low output) and observe the change in port C.
5Use OUT 02H to drive the low nibble of port C and confirm which LEDs light.

Tasks

Echo switches to LEDs
Mirror the switch pattern on the LED bank continuously.
Control port C
Use the bit-set/reset mode to turn a single PC0 line on and off.
Reconfigure the ports
Switch port A to output and demonstrate data flowing the other way.

Self-check

The switch pattern appears unchanged on the LED bank.
Port C low-nibble output matches the OUT 02H writes.
The student can derive a control word from a port-direction description.

Environment

8085 assembler8255 port panelswitch and LED simulationcontrol word builder