EGR 2131 Unit 6
Combinational Building Blocks

Read Mano & Ciletti, Sections 4.9
to 4.15.
Midterm Exam next week.
 Homework #6 and Lab #6 due
next week.

Enable Pins
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Many of the chips we’ll study have enable
inputs. Depending on the logic level at this
pin, the chip is either enabled or disabled.
When the chip is enabled, it performs its
intended function and the outputs behave as
you would expect.
When the chip is disabled, then (usually) all
outputs are forced to their inactive state,
regardless of the other inputs to the chip.
Common names for enable pins include EN,
G (for “gate”), and CS (for “chip select”).
Active-High versus Active-Low Pins
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
Each input pin and output pin on a chip
is either active-high or active-low.
In a logic symbol:


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Active-low pins are marked with a bubble or
triangle.
Active-high pins have no bubble or triangle.
Active-high pins: the pin is active when
there’s a HIGH on that pin.
Many chips have active-low pins: the
pin is active when there’s a LOW on
that pin.
Example of a Chip With Enable Inputs and
Some Active-Low Pins: 74154 Decoder
From Floyd, p. 306
From Texas Instrument datasheet
Review: Useful Building-Block
Circuits

Here are some kinds of digital circuits we’ve
studied or will study in the weeks ahead:
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Adders
Comparators
Decoders
Encoders
Code converters
Multiplexers
Latches & Flip-flops
Shift registers
Counters
Memory
Chapter 4
Combinational
Chapter 5
Chapter 6
Sequential
Chapter 7
Decoders, Encoders, & Code Converters
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
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Decoders convert a binary code into
a single active output representing the
code’s value.
Encoders generate a coded output
from a single active input line.
Code converters take one input code
(such as BCD) and convert it to
another code (such as binary).
Decoders
A decoder is a logic circuit that detects the presence of a specific
combination of bits at its input. Two simple decoders that detect the
presence of the binary code 0011 are shown. The first has an active
HIGH output; the second has an active LOW output.
A0
A1
A0
X
A1
X
A2
A2
A3
A3
Active HIGH decoder for 0011
Active LOW decoder for 0011
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Decoders
Assume the output of the decoder shown is a
logic 1. What are the inputs to the decoder?
A0 = 0
A1 = 1
1
A2 = 0
A3 = 1
© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved
Decoders
IC decoders have multiple outputs to decode any
combination of inputs. For example the hex decoder
shown here has 16 outputs – one for each combination of
binary inputs.
Bin/Dec
For the input shown,
what is the output?
1
4-bit binary
input
1
0
1
A0
A1
A2
A3
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
1
1
1
1
1
1
1
1
1
1
1
0
1
1
1
1
Decimal
outputs
© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved
Decoders
X/Y
A specific integrated circuit
decoder is the 74154, a 4-to-16
decoder. It includes two active
LOW chip select lines which must
be at the active level to enable the
outputs. These lines can be used to
expand the decoder to larger inputs.
A0
A1
A2
A3
CS1
CS2
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
1
2
4
8
&
15
EN
74154
© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved
Octal Decoder
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3 data input pins for input code.
8 output pins.
Also called 1-of-8 decoder or
3-line-to-8-line decoder.
May have other inputs and outputs
too, such as enable inputs.
Example chip: 74138
Hex Decoder
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4 data input pins for input code.
16 output pins.
Also called 1-of-16 decoder or
4-line-to-16-line decoder.
May have other inputs and outputs
too, such as enable inputs.
Example chip: 74154
BCD Decoder
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4 data input pins for input code.
10 output pins.
Also called 1-of-10 decoder or
4-line-to-10-line decoder.
May have other inputs and outputs
too, such as enable inputs.
Example chip: 7442
Encoders
An encoder accepts an active logic level on one of its
inputs and converts it to a coded output, such as BCD or
binary.
The basic logic diagram is shown.
This encoder has an input for each
decimal digit, and four outputs that
represent the binary code for the
active input digit.
There is no zero input because the
outputs are all LOW when the input
is zero.
1
A0
2
3
4
5
6
7
8
A1
A2
A3
9
© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved
Encoders
Show how the BCD encoder converts the decimal
number 3 into a BCD 0011.
The top two OR gates have ones as indicated with
the red lines. Thus the output is 0011.
1 0
1
2 0
1
3
1
4
5
6
7
8
9
0
0
0
0
0
0
0
0
A0
A1
A2
A3
© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved
Encoders
The 74147 is an example of an IC encoder. It is has ten
active-LOW inputs and converts the active input to an
V
active-LOW BCD output.
CC
(16)
This device offers additional
flexibility in that it is a priority
encoder. This means that if more
than one input is active, the one
with the highest order decimal
digit will be active.
Decimal
input
(11)
(12)
(13)
(1)
(2)
(3)
(4)
(5)
(10)
HPRI/BCD
1
2
3
4
5
6
7
8
9
74HC147
The next slide shows an application …
1
2
4
8
(9)
(7)
(6)
(14)
BCD
output
(8)
GND
© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved
VCC
Encoders
Keyboard
encoder
R7
7
R8
8
R9
9
HPRI/BCD
R4
4
R5
5
R1
1
0
6
R2
2
R0
R6
R3
1
2
3
4
5
6
7
8
9
1
2
4
8
BCD complement of
key press
74HC147
3
The zero line is not needed by the
encoder, but may be used by other
circuits to detect a key press.
© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved
BCD Encoder
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10 input pins.
4 output pins for output code.
Also called 10-line-to-4-line
encoder.
May have other inputs and outputs
too, such as enable inputs.
Example chip: 74147
Octal Encoder
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8 input pins.
3 output pins for output code.
Also called 8-line-to-3-line encoder.
May have other inputs and outputs
too, such as enable inputs.
Example chip: 74148
Different Numeric Codes
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
Several different codes exist for using
1s and 0s to represent positive
integers.
Standard binary code
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Binary-coded decimal (BCD)
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Example: In standard binary, 15 is 1111.
Example: In BCD, 15 is 0001 0101.
Gray code

Example: In Gray code, 15 is 1000.
Four-Bit Gray Code
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The key feature
of Gray code is
that only one bit
changes when
we increase a
number by one.
This is not true
of standard
binary.
Why is Gray Code Useful?
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
Gray code is used for rotary encoders
that sense the angular position of a
shaft or axle.
From Wikipedia article on rotary encoders:
Standard 3-bit binary code: no good!
3-bit Gray code: better!
Code Converters
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If a digital system needs to handle
numbers using two different codes, it
needs circuitry to convert between the
two codes.
Examples of code converters:


74184 BCD-to-binary and binary-to-BCD
converter
Binary-to-Gray code or Gray-code-tobinary converters (see next slide)
Gray Code/Binary Converters
Figure 8-40. Binary-to-Gray-code converter
Figure 8-41. Gray-code-to-binary converter
Multiplexers
A multiplexer (MUX) selects one data line from two or
more input lines and routes data from the selected line to
the output. The particular data line that is selected is
determined by the select inputs.
Two select lines are shown
here to choose any of the
four data inputs.
Select
inputs
Which data line is selected if
S1S0 = 10? D2
S0
S1
D0
Data D1
inputs D2
D3
0
1
MUX
0
1
0
1
2
3
Data
output
© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved
Some Multiplexer Chips
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74150
74151
74153
74157
(16-input MUX)
(8-input MUX)
(dual 4-input MUX)
(quad 2-input MUX)
Demultiplexers
A demultiplexer (DEMUX) performs the opposite function
from a MUX. It switches data from one input line to two or
more data lines depending on the select inputs.
The 74LS138 was introduced
previously as a decoder but can also
serve as a DEMUX. When
connected as a DEMUX, data is
applied to one of the enable inputs,
and routed to the selected output
line depending on the select
variables. Note that the outputs are
active-LOW as illustrated in the
following example…
DEMUX
Data
select
lines
Enable
inputs
A0
A1
A2
G1
G2A
G2B
Y0
Y1
Y2
Y3
Y4
Y5
Y6
Y7
Data
outputs
74LS138
© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved
Demultiplexers
Determine the outputs, given the
inputs shown.
The output logic is opposite to the input
because of the active-LOW convention. (Red
shows the selected line).
DEMUX
Data
select
lines
Enable
inputs
A0
A1
A2
G1
G2A
G2B
74LS138
Y0
Y1
Y2
Y3
Y4
Y5
Y6
Y7
Data
outputs
A0
A1
A2
G1
G2A LOW
G2B LOW
Y0
Y1
Y2
Y3
Y4
Y5
Y6
Y7
© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved
Some Demultiplexer Chips
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74138 (3-line to 8-line Decoder/DEMUX)
74154 (4-line to 16-line Decoder/DEMUX)
74139 (dual 2-line to 4-line Decoder/DEMUX)
Choices in Quartus II
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In Quartus II you have your choice of generic
muxes, decoders, adders, and so on, or ones
having the same features as the ones in
74XX-series chips.
Type
Sample
To use the latter, you must
Chips
know the 74XX number
3-to-8
74138
for the chip you want.
decoder
Some examples are listed
Dual 4-input
74153
here.
MUX
Recall that Wikipedia has
4-bit full
7483
adder
a complete list of 74XX
chips (not all of which
are available in Quartus.)
Choices in Quartus II (Cont’d.)
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Generic:

74XX:
A 74153 chip contains two 4-input
muxes, each of which has its own
inputs named GN, C0, C1, C2, C3,
and its own output named Y. (The
A and B inputs are shared between
the two muxes.)
Choices in Quartus II (Cont’d.)
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The choices on the previous slide
assumed that you’re using schematic
entry to draw a diagram of your
design.
Another alternative is to use text entry
with VHDL. Later we’ll see examples
of VHDL code for decoders, encoders,
multiplexers, and so on.