Transcript The BCD Code

Digital Electronics
Principles & Applications
Seventh Edition
Roger L. Tokheim
Chapter 6
Encoding, Decoding, and
Seven-Segment Displays
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
INTRODUCTION
• The BCD Code
• Decoders
• The Excess-3 Code
• BCD-to-SevenSegment Decoder/
Driver
• The Gray Code
• The ASCII Code
• Encoders
• Seven-segment
Displays
• Seven-segment LED
Displays
• Liquid Crystal
Displays
• Vacuum Fluorescent
Displays
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
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
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
QUIZ
Convert the BCD coded number
1000 0111 0001 into decimal.
BCD Coded Number
Decimal Number
1000 0111 0001
8
7
1
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
QUIZ
Convert the decimal number
350 to its BCD equivalent.
Decimal Number
BCD Coded Number
3
5
0
0011 0101 0000
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
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
0
1
2
3
4
5
6
7
8
9
Binary +3 Excess-3
0000 0011 0011
0001 0011 0100
0010 0011 0101
0011 0011 0110
0100 0011 0111
0101 0011 1000
0110 0011 1001
0111 0011 1010
1000 0011 1011
1001 0011 1100
Sample Problem:
Decimal
Excess-3
3
5
9
0110 1000 1100
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
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
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
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
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
00000
00001
00011
00010
00110
00111
00101
00100
01100
01101
01111
01110
01010
01011
01001
01000
11000
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
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
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
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)
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
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
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
Electronic Encoder - Decimal to BCD
BCD output
Decimal input
7
5
3
00
11
01
0
Decimal
to
BCD
Encoder
0
• Encoders are available in IC form.
• This encoder translates from decimal
input to BCD output.
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
Seven Segment Displays
LED (light emitting diode) display
• High power consumption
• Viewing best under low light conditions
• Commonly red but available in other colors
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
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
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
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)
True
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)
True
3. The seven-segment LED display is best viewed in
very bright sunlight. (True or False)
False
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
7-Segment LED Display Operation
Inputs
7-segment code
7-segment
b & c inputsdisplay
active
Note
reads
that
therefore
decimal
inputs 1
are
(segments
segments
“active-LOW”
bb &
& cc
lightare
on lit)
display
H
L b
L c
H
H
H
H
a
+5V
g
Limiting resistors
Common anode
Cathode terminals (a thru g)
7-segment LED display
(common anode)
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
QUIZ
#1-What
Whatiswill
the decimal
the 7-segment
LED
QQ#4#2the be
decimal
outputoutput
on the on
display
for the new
set display?
of inputs?
#3-
Inputs
7-segment code
LH
LH
L
H
L
H
L
H
L
H
L
a
b
b
c
d
e
f
g
a
+5V
g
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
Decoders: BCD to
7-Segment Decoder/Driver
Decimal output
LED
BCD input
0 10 01 01
BCD-to7-Segment
Decoder/
Driver
• 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.
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
QUIZ
Q #5#1- What is the decimal output from the decoder that
#2#3#4appears on the 7-segment display?
Answer: 6
3
0
9
7
BCD input
Decimal output
010 100 010 0110
BCD-to7-Segment
Decoder/
Driver
?
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
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.
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
Liquid Crystal Display
Metalized segments
Polarizer
Conductive
pattern on glass
Contacts
Liquid crystal
Metalized layer
Glass backplane
Polarizer
100 Hz
out-of-phase signal
activates segment
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
QUIZ
1. While VF and LED displays emit light,
LCDs __________ (control available,
sublimate red) light.
control available
2. The nematic fluid sandwiched between the
glass plates of a LCD is also called _______
_______ (green phosphor, liquid crystal).
liquid crystal
3. Monochrome twisted-nematic field-effect
LCDs display __________ (dark, white)
figures on a silvery background.
dark
4. Liquid crystal displays are difficult to read
under low light conditions however most
laptop computer flat screens are backlighted
True
to add brightness. (True or False)
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
Driving an LCD Display
BCD Input
0111
a
b
c
LCD
d
driver e
f
g
Phase
100 Hz Clock
Common
The LCD segments
with out-of-phase signals
are active
(a, b, & c are active)
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
QUIZ
Q#1:
Q#2:
What
isisthe
the
output
on
the
LCD
display?
Q#3: What
Whatis
theoutput
outputon
onthe
theLCD
LCDdisplay?
display?
BCD Input
A
B
LCD C
Driver D
E
F
G
ANS:
2
ANS:
ANS: 05
?
LCD
Common
100 Hz Clock
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
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-ofsale terminals and some desktop calculators.
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
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
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
Vacuum Fluorescent Display
Cathodes (heaters)
Grid
KG f a
e
g d c
b
K
Plates
Glass
back
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
QUIZ
Q#1atsection
A are
called
or
Q#2Thewires
segments
at B
are
___.
Q#3-The
Thethin
screen
like
at
C called
isheaters
called
the___.
___.
A
ANS:
Cathodes
ANS:
Plates
ANS:
Grid
B
C
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
Operating a VF 7-Segment Display
+12 V
Pa
Pb
Pc
0V
Pd
Pe
Pf
Pg
Plates
+12 V
Cathodes
(heaters)
Grid
To light a segment:
1. Heaters must be hot
2. Grid must be at +12V
3. Plate must be at +12V
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
QUIZ
Q#1Part
A
isthe
theheaters
___.at +12V.
Q#2Q#4Part
The
B points
grid
isat
normally
or ___.
Q#5To
Q#3light
Parts
a
plate
C
are
it
must
the
___.
bebegrounded.
Q#6- To light a plate it must
at +12V.
(T
or
F)
(T
(Tor
orF)
F)
ANS:cathodes
grid
ANS:
ANS:
plates
ANS:
True
ANS:
ANS: False
True
C
A
B
Schematic diagram
of VF display with
heaters connected
to battery.
©2008 The McGraw-Hill Companies, Inc. All rights reserved.
REVIEW
• The BCD Code
• Decoders
• The Excess-3 Code
• BCD-to-SevenSegment Decoder/
Driver
• The Gray Code
• The ASCII Code
• Encoders
• Seven-segment
Displays
• Seven-segment LED
Displays
• Liquid Crystal
Displays
• Vacuum Fluorescent
Displays
©2008 The McGraw-Hill Companies, Inc. All rights reserved.