©2008 the mcgraw-hill companies, inc. all rights reserved. digital electronics principles &...

©2008 The McGraw-Hill Companies, Inc. All rights reserved.

Digital ElectronicsDigital Electronics

Principles & ApplicationsPrinciples & ApplicationsSeventh EditionSeventh Edition

Chapter 6Encoding, Decoding, andSeven-Segment Displays

Roger L. Tokheim



INTRODUCTION

• The BCD Code

• The Excess-3 Code

• The Gray Code

• The ASCII Code

• Encoders

• Seven-segment

Displays

• Seven-segment LED

Displays

• Decoders

• BCD-to-Seven-

Segment Decoder/

Driver

• Liquid Crystal

Displays

• Vacuum Fluorescent

Displays


The 8421 BCD Code

• BCD stands for Binary-Coded Decimal.

• A BCD number is a four-bit binary group that represents one of the ten decimal digits 0 through 9.

Example:

Decimal number 4926 4 9 2 6

8421 BCD coded number 0100 1001 0010 0110


Convert the BCD coded number

1000 0111 0001 into decimal.

QUIZ

Decimal Number 8 7 1

BCD Coded Number 1000 0111 0001


Convert the decimal number 350 to its BCD equivalent.

Decimal Number 3 5 0

BCD Coded Number 0011 0101 0000

QUIZ


The Excess-3 Code• Add 3 to each digit of decimal and

convert to 4-bit binary form• A BCD code (not 8421 BCD)

Decimal Binary +3 Excess-3 0 0000 0011 0011 1 0001 0011 0100 2 0010 0011 0101 3 0011 0011 0110 4 0100 0011 0111 5 0101 0011 1000 6 0110 0011 1001 7 0111 0011 1010 8 1000 0011 1011 9 1001 0011 1100

Decimal 3 5 9

Sample Problem:

Excess-3 0110 1000 1100


QUIZ

1. To form an excess-3 coded number, decimal 3 is added to the decimal number and this is converted to its 4-bit binary code. (True or False) True

2. The excess-3 BCD number 1010 0100 equals __________ in decimal. 71

3. The decimal number 428 equals what excess-3 number. 0111 0101 1011


The Gray Code

• The Gray code’s most

important characteristic is that

only one digit changes as you

increment or decrement the

count.

• The Gray code is commonly

associated with input/output

devices such as an optical

encoder of a shaft’s angular

position.

• The Gray code is NOT a BCD

code.

Decimal Gray code

0 00000 1 00001 2 00011 3 00010 4 00110 5 00111 6 00101 7 00100 8 01100 9 01101 10 01111 11 01110 12 01010 13 01011 14 01001 15 01000 16 11000


QUIZ

1. The Gray code is not a type of BCD code. (True or False)

2. The most important characteristic of the ____________ (BCD, Gray code) is that only one digit changes state as you increment or decrement the count.

3. The Gray code is commonly associated with input/output devices such as an optical encoder of a shaft’s angular position. (True or False)

True

Gray code

True


The ASCII Code• ASCII is an acronym for American

Standard Code for Information Interchange

• Represents numbers, letters, punctuation marks and control characters

• Standard ASCII is a 7-bit code (127 characters)

• Extended ASCII (IBM ASCII), an 8-bit code, is also very popular

• Extended ASCII adds graphics and math symbols to code (total of 256 symbols)


QUIZ

1. A common 7-bit code used to represent numbers, letters, punctuation marks, and control characters is known by the acronym __________.

2. The acronym ASCII stands for __________________________.

3. Extended ASCII code is a(n) __________ (8-bit, 10-bit) code which adds graphic and math symbols to ASCII for a total of 256 symbols.

ASCII

American Standard Code for Information

Interchange

8-bit


Electronic Encoder - Decimal to BCD

0

Decimalto

BCDEncoder

BCD output

Decimal input0 0 0 0

5

0 1 0 1

7

0 1 1 1

3

0 0 1 1

• Encoders are available in IC form.

• This encoder translates from decimal input to BCD output.


Seven Segment Displays

LCD (liquid crystal display)• Commonly dark characters on a silvery background• Very low power consumption• Viewing only OK in good light• Backlighted displays can be viewed in low light

LED (light emitting diode) display• High power consumption• Viewing best under low light conditions• Commonly red but available in other colors

VF (vacuum fluorescent) display• Commonly blue-green display (without filters)• Medium power consumption• Viewing good from dark to lighter conditions• Based on vacuum tube triode technology


QUIZ

1. The type of 7-segment display based on vacuum tube technology which usually has blue-green characters and can be viewed in dark to lighter conditions is the vacuum fluorescent display (VF). (True or False)

2. The type of 7-segment display that shows dark characters on a silvery background, uses very little power, and is good for viewing in bright light is called the liquid crystal display (LCD). (True or False)

3. The seven-segment LED display is best viewed in very bright sunlight. (True or False)

True

True

False


7-Segment LED Display Operation

Limiting resistors

7-segment LED display(common anode)

HL bL cHHHH

+5Vaa

gg

Inputs7-segment code

Common anode

Cathode terminals (a thru g)

b & c inputs activetherefore

segments b & clight on display

Note that inputs are “active-LOW”

7-segment display reads decimal 1(segments b & c

are lit)


L aL bL cHHHH

+5Va

g

Inputs7-segment code

Q #1- What will be the decimal output on the 7-segment LED display?Q #2- What is the decimal output on the display for the new set of inputs?

L aHL cL dHL fL g

L aL bL cL dL eL fL g

Q #3- What is the decimal output on the display for the new set of inputs?Q #4- What is the decimal output on the display for the new set of inputs?

HL bL cHHL fL g

QUIZ


BCD-to-7-SegmentDecoder/

Driver

Decoders: BCD to 7-Segment Decoder/Driver

BCD input

0 0 0 00 0 0 0

Decimal outputLED

0 0 0 10 0 0 10 0 1 00 0 1 00 0 1 10 0 1 10 1 0 00 1 0 0

• Electronic decoders are available in IC form.• This decoder translates from BCD to decimal.• Decimals are shown on an 7-segment LED display.• This IC also drives the 7-segment LED display.


BCD-to-7-SegmentDecoder/

Driver

BCD inputDecimal output

0 0 1 1

?

Q #1- What is the decimal output from the decoder that appears on the 7-segment display?

Answer: 3


0 0 0 0

Answer: 0


1 0 0 1

Answer: 9


Answer: 7

0 1 1 1


Answer: 6

0 1 1 0

QUIZ


Liquid Crystal Displays• LCDs control available light (do not generate light)

• Nematic fluid (also called liquid crystal fluid) is sandwiched between two glass plates

• Monochrome twisted-nematic field-effect LCDs display dark figures on a silvery background.

• LCDs are commonly used on watches, calculators, and laptops. LCD monitors and TVs are becoming common.

• Color LCDs use either passive-matrix or active-matrix technology.

• Active-matrix color LCD screens are more expensive.

• Active-matrix color LCD screens use thin-film transistors (TFT) and red, green, blue, and white filters.

• Most laptop screens are backlighted for brightness.


100 Hzout-of-phase signalactivates segment

Metalized segments

Liquid crystal

Metalized layer

Glass backplane

Polarizer

Contacts

Liquid Crystal Display

Polarizer

Conductivepattern on glass


QUIZ

1. While VF and LED displays emit light, LCDs __________ (control available, sublimate red) light.

2. The nematic fluid sandwiched between the glass plates of a LCD is also called _______ _______ (green phosphor, liquid crystal).

3. Monochrome twisted-nematic field-effect LCDs display __________ (dark, white) figures on a silvery background.

4. Liquid crystal displays are difficult to read under low light conditions however most laptop computer flat screens are backlighted to add brightness. (True or False)

control available

liquid crystal

dark

True


abcdefg

CommonPhase

100 Hz Clock

LCDdriver

The LCD segmentswith out-of-phase signals

are active(a, b, & c are active)

Driving an LCD Display

BCD Input

0 1 1 1


QUIZ

Q#1: What is the output on the LCD display?

100 Hz Clock

?BCD Input A

BCDEFG

LCDDriver

LCD

Common

ANS: 2Q#2: What is the output on the LCD display? ANS: 0Q#3: What is the output on the LCD display? ANS: 5


Vacuum Fluorescent Displays

• VF displays are commonly used in automobiles because they operate at the proper voltages, have a pleasing blue-green color, are easy to see even when viewed under lower light conditions, and will operate at wide extremes of temperature.

• VF displays are based on an older triode vacuum tube technology.

• VF displays are also commonly used in point-of-sale terminals and some desktop calculators.


QUIZ

1. A vacuum fluorescent display is based on an older vacuum tube technology and is commonly referred to as a _____ _________ (TFT display, VF display).

2. One of the most common applications of VF displays is in _________________ (flat-screen laptop computer screens, automobiles).

3. The VF display has the advantage of being easily readable in either light or dark conditions. (True or False)

VF display

automobiles

True


Vacuum Fluorescent Display

K G f a e g d c b K

Cathodes (heaters)

Grid

GlassbackPlates


QUIZ

B

C

Q#1- The thin wires at A are called heaters or ___.

AANS: Cathodes

Q#2- The segments at B are called ___.

ANS: Plates

Q#3- The screen like section at C is called the ___.

ANS: Grid


Pa Pb Pc Pd Pe Pf Pg

+12 V

+12 V 0 V

To light a segment:1. Heaters must be hot

2. Grid must be at +12V 3. Plate must be at +12V

Operating a VF 7-Segment Display

PlatesPlatesPlatesPlates

Cathodes(heaters)Cathodes(heaters)Cathodes(heaters)Cathodes(heaters)GridGridGridGrid


QUIZ

A

B

C

Q#1- Part A is the ___.

Schematic diagram of VF display with heaters connected to battery.

ANS: gridQ#2- Part B points at the heaters or ___.ANS: cathodesQ#3- Parts C are the ___.

ANS: platesQ#4- The grid is normally at +12V.

(T or F) ANS: TrueQ#5- To light a plate it must be grounded.

(T or F) ANS: FalseQ#6- To light a plate it must be at +12V.

(T or F) ANS: True


REVIEW

• The BCD Code

• The Excess-3 Code

• The Gray Code

• The ASCII Code

• Encoders

• Seven-segment

Displays

• Seven-segment LED

Displays

• Decoders

• BCD-to-Seven-

Segment Decoder/

Driver

• Liquid Crystal

Displays

• Vacuum Fluorescent

Displays

©2008 the mcgraw-hill companies, inc. all rights reserved. digital electronics principles &...

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