Question

Subject: computer architecture and microprocessor

Case Analysis Read the case situation given below and answer the questions that ollow: MPU 8085 communicates with peripherals through 8255 in different modes. For this it has to take following three steps: Determining the addresses of ports A, B, and C and of the control register according to the chip select logic and address ines Ao Ai. Writing a control word in the control register Writing I/O instructions o communicate with peripherals through port A, B and C... Answer the following questions: a) What do you understamd by BSR mode and control word? b) Mention different modes available in 8255 and write about its characteristics. c) Identify the port address, generate control word to configure port A and port Cu as output ports and port B and port CL as input ports. d) Write a progra to read DIP witches and display the reading from port B at port A and from port CL at port Cu.

Answer 1

Handshaking

The making of inter relation between slower peripheral device and microprocessor is called handshaking.

Handshaking Signal

Before making the inter-relation between peripheral device and microprocessor the PPI send some signals to microprocessor and peripheral device to perform the process, these signals are called handshaking signal. 8255-based devices that perform handshaking support following handshaking signals:

1) Strobe Input (STB) 2) Input Buffer Full (IBF) Digital Input operation 3) Output Buffer Full (OBF) 4) Acknowledge Input (ACK) Digital Output operation 5) Interrupt

Read OperationSTB

goes low indicates that data are loads into port latch.

IBF Becomes high (at high to low transition of STB) indicates that input latch contains data.

INTR Becomes high (at low to high transition of STB) uPgoes interrupt subroutine to read data.

RD becomes low.IBFbecomes low when read complete, RD becomes high and IBF goes low.

IBF low means input latch has no data (Read complete)

Handshaking Signal (Continued) Data Bus Data Bus Input IBF Device 8086 RD 8255 INTR STB STB IBF (Buffer full INTR Interrupt requested) RD Port Data strobed into pornt Data read by microprocessor

It consists of three 8-bit bidirectional I/O ports i.e. PORT A, PORT B and PORT C. We can assign different ports as input or output functions.

Block diagram –

It consists of 40 pins and operates in +5V regulated power supply. Port C is further divided into two 4-bit ports i.e. port C lower and port C upper and port C can work in either BSR (bit set rest) mode or in mode 0 of input-output mode of 8255. Port B can work in either mode or in mode 1 of input-output mode. Port A can work either in mode 0, mode 1 or mode 2 of input-output mode.

It has two control groups, control group A and control group B. Control group A consist of port A and port C upper. Control group B consists of port C lower and port B.

Depending upon the value if CS’, A1 and A0 we can select different ports in different modes as input-output function or BSR. This is done by writing a suitable word in control register (control word D0-D7).

S’	A1	A0	Selection	Address
0	0	0	PORT A	80 H
0	0	1	PORT B	81 H
0	1	0	PORT C	82 H
0	1	1	Control Register	83 H
1	X	X	No Seletion	X

Pin diagram –

• PA0 – PA7 – Pins of port A
• PB0 – PB7 – Pins of port B
• PC0 – PC7 – Pins of port C
• D0 – D7 – Data pins for the transfer of data
• RESET – Reset input
• RD’ – Read input
• WR’ – Write input
• CS’ – Chip select
• A1 and A0 – Address pin

Write a control word to configure port A as input port in mode 0 and port B in mode 1 as output port.

Solution:The control word is 94H.N.B. D0 and D3 are low if port C is used as output or if unused

solution of d) Program

PPICR EQU 00FBH

PPIC EQU 00FAH

PPIB EQU 00F9H

PPIA EQU 00F8H

MOV AL, 83HOUT

PPICR , ALIN

AL, PPIBOUT

PPIA, ALIN

AL, PPICAND

AL, 0FH

MOV CL,

04HROL

AL, CLOUT

PPIC, AL

HLT