BY
Ramesh H R
Asst.Prof.
Dept of TE
TOPICS
8255 Programmable peripheral interface(PPI)
Keyboard Interfacing
Stepper motor Interfacing
Alphanumeric display Interfacing
SESSION 1
Programmable peripheral Interface
TOPICS:
1. Introduction
2. Block diagram
3. Modes of operation
Control word format
I/O mode
BSR mode
PPI – Programmable Peripheral Interface
1. It is an I/O port chip used for interfacing I/O devices with microprocessor
2. Very commonly used peripheral chip.
3. It has 24 I/O pins which can be grouped in three 8-bit parallel ports: Port A, Port B and Port
C(two 4 bits port Port Cupper and Port C lower. )
4. The 8255 ,primarily can be programmed in two basics modes:
1. Bit set/reset (BSR) mode.
2. I/O mode.
The I/O mode is divided into three modes:
mode 0: simple Input/output
mode 1: Input /Output with handshake
mode2: Bi-directional I/O transfer.
The BSR mode is used to set or reset the bits in Port C
Block diagram
Data bus buffer:
This tri state bi-directional buffer is used to interface the internal bus of 8255 to the
system data bus.
Input or output instructions executed by the CPU either read data from or write data into
the buffer.
Output data from the CPU to the ports or control register and input data to the CPU from
the ports or status register are all passed through the buffer.
Control logic
• The control logic block accepts control bus signals as well as inputs from the address bus
and issues commands to the individual group control blocks(Group A and Group B
control).
• It issues appropriate enabling signals to access the required data/control words or status
word.
Port and register select summary
I/O modes
Mode 0: Simple Input or Output
In this mode, ports A, B are used as two simple 8-bit I/O ports
port C as two 4-bit ports.
Each port can be programmed to function as simply an input port or an
output port. The input/output features in Mode 0 are as follows.
1. Outputs are latched.
2. Inputs are not latched.
3. Ports don’t have handshake or interrupt capability.
Mode 1: Input or Output with Handshake
In this mode, handshake signals are exchanged between
the MPU and peripherals prior to data transfer.
The features of the mode include the following:
1. Two ports (A and B) function as 8-bit I/O ports.
They can be configured as either as input or output ports.
2. Each port uses three lines from ort C as handshake signals.
The remaining two lines of Port C can be used for simple I/O operations.
3. Input and Output data are latched.
4. Interrupt logic is supported.
Mode 2: Bidirectional Data Transfer
This mode is used primarily in applications such as data transfer
between two computers.
In this mode, Port A can be configured as the bidirectional port
Port B either in Mode 0 or Mode 1.
Port A uses five signals from Port C
as handshake signals for data transfer.
The remaining three signals from port C can be used either
as simple I/O or as handshake for port B.
Control word formats
Bit Set/Reset mode:
E.g:- Program to blink port C bit 0
Flow chart: *Program:
mov dx,ctrl
back:mov al,01h
out dx,al
call delay
mov al,00h
out dx,al
jmp back
Control word format for I/O mode
Examples:
1. write a program to initialize 8255 in the
configuration given below.
Port A:simple input
Port B : simple output
Port CL : output
Port C U: input
Assume address of the control word register of
8255 is 83 h.
MOV AL,98H
OUT 83H,AL
Ex. 1: Configure Port A as i/p in Mode 0, Port B as
o/p in mode 0, Port C (Lower) as o/p and Port C
(Upper) as i/p ports.
Port A can work in Mode 0, Mode 1, or Mode 2
Port B can work in Mode 0, or Mode 1
Port C can work in Mode 0 only, if at all
Port A, Port B and Port C can work in Mode 0
Port A and Port B can work in Mode 1
Only Port A can work in Mode 2
