Unit - IV_ppi

8/8/2019 Unit - IV_ppi

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Unit - IV

8255 PPI

- Programmable Peripheral

Interfacing


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Programmable Peripheral Interface

PPI expands number of parallel ports

Intel 8255 has three 8-bit ports: Port A, B, C and a Control

register

Ports A and B can be programmed as either input or output. Ports A and C can be used as bidirectional

Port C can be programmed as a pair of 4-bit bidirectional

control ports or individual lines can be set or reset


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PPI cont

Control register controls the overall operation of 8255

All three ports A, B and C are grouped into two

Group BGroup A

Port A Upper C Port BLower C


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PPI cont..

it can operate in two basic modes

Bit set/reset mode (BSR)

I/O Mode:

mode 0,

mode 1,

mode 2

Bit 8 of Control register decides between 3-

mode operation or bit set/reset operation


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Operation modes

8255 has three modes:

- mode 0: basic input-output

- mode 1: strobed input-output

- mode 2: strobed bidirectinal bus I/O

In mode 0

- two 8-bit ports and two 4-bit ports

- any port can be input or output

- Outputs are latched, inputs are not latched


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Mode 0



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Operation mode 1

In mode 1:

-three ports are divided into two groups

-each group contains one 8-bit port and one 4-bit control/data

port

- 8-bit port can be either input or output and both latched- 4-bit port used for control and status of 8-bit data port


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Mode 1

Group BGroup A



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In mode 2

- only port A is used

- port A becomes an 8-bit bidirectional bus

- port C acts as control port (only pins PC3-PC7 are used)

Operation mode 2


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Mode 2

Port A C7-C3 Port B


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Address mapping of ports and control register

8255 PPI

8255 control registerp $106F

Port Ap $106C

Port Bp $106D

Port Cp $106E


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Block Diagram of 8255


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Pin Diagram


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Control word register :


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on ro reg s er

D7 D6 D5 D4 D3 D2 D1 D0

7 6 5 4 3 2 1 0

Port A 1 Input 0 output

Mode select: 00 mode 0;

01 mode 1; 0x mode 21 mode select

0 bit set/reset

Port C(U) 1 Input 0 output

Mode select: 0 mode 0;1 mode 1

Port B 1 Input 0 output

Port C(L) 1 Input

0 output

Group A

Group B


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InitialisingControl Register

Control word: Port A input and port B output and both port in mode 0

A B

7 6 5 4 3 2 1 0

1 0 0 1 x 0 0 x


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Programming 8255

Mode 0:

Ports A, B, and C can be individually programmed as input or output ports

Port C is divided into two 4-bit ports which are independent from each other

Mode 1:

Ports A and B are programmed as input or output ports

Port C is used for handshaking

PA[7:0]

STBAIBFAINTRAPC3

PC5PC4

PB[7:0]STBBIBFBINTRBPC0

PC1PC2

PC6, 7

8255

PA[7:0]

OBFAACKAINTRAPC3

PC6PC7

PB[7:0]OBFBACKBINTRBPC0

PC1PC2

PC4, 5

8255


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Input Control signals:


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Output Control Signals:


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Programming 8255

Mode 2:

Port A is programmed to be bi-directional Port C is for handshaking

Port B can be either input or output in mode 0 or mode 1

PA[7:0]

OBFAACKA

INTRA

PC4

PC6PC7

STBAIBFA

PC0

PC3

PC58255

PC0

PC0

PB[7:0]

In Out

In Out

In Out

Mode 0

STBB OBFBIBFB ACKBINTRB INTRB

Mode 1

1. Can you design a decoder for an 8255 chip such that its base address is 40H?

2. Write the instructions that set 8255 into mode 0, port A as input, port B as output,

PC0-PC3 as input, PC4-PC7 as output ?


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Control signals:


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Timing diagram is a combination of the Mode 1Strobed Input

and Mode 1Strobed Output Timing diagrams.


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Interfacing 8255 to 8086 in I/O mapped I/O

D0-D7D0-D7

A0

A1A2

A1

RD

WRWR

M/IO

RST CS

Port A

Port B

Port C

A3

A4A5

A6

A7

RD

A0


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D0-D7D0-D7

A0

A1A2

A1

Interfacing 8255 to 8086 in Memory mapped I/O

A3

A4

A5

A6

A7

A19

WR

M/IORD

A0

RD

RST

WR

CS

Port A

Port B

Port C


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Example Program

Write a program for this traffic signal problem.

ort8255-

68HC11-

based QP

0

1

2

3

4

5

6

7

R

+

-

+

-


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Summary

8255 has very friendly programming features

Data can be sent and read to and from outer world via ports A,B

and C

Suitable for advanced applications

Documents

Unit - IV_ppi