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Digital Electronics
Tutorial: Combinational Logic Design
Examples
Solutions
ELEC1041 - Combinational Logic Examples - 1
Problem #1
Develop a minimized Boolean implementation of a “ones count” circuit
that works as follows. The subsystem has four binary inputs A, B, C,
D, and generates a 3-bit output, XYZ. XYZ is 000 if none of the
inputs are 1, 001 if one input is 1, 010 if two are one, 011 if three
inputs are 1, and 100 if all four inputs are 1.
(a) Draw the truth tables for XYZ (A, B, C, D).
(b)
Minimize the functions X, Y, Z using 4-variable K-maps. Write
down the Boolean expressions for the minimized Sum of Products
form of each function.
(c)
Repeat the minimization process, this time deriving Product of
Sums form.
ELEC1041 - Combinational Logic Examples - 2
Problem #1 Solution (1/6)
(a) Draw the truth tables for XYZ (A, B, C, D).
ELEC1041 - Combinational Logic Examples - 3
Problem #1 Solution (2/6)
(b)
Minimize the functions X, Y, Z using 4-variable K-maps. Write down the Boolean
expressions for the minimized Sum of Products form of each function.
ELEC1041 - Combinational Logic Examples - 4
Problem #1 Solution (3/6)
ELEC1041 - Combinational Logic Examples - 5
Problem #1 Solution (4/6)
(c)
Repeat the minimization process, this time deriving Product of
Sums form.
ELEC1041 - Combinational Logic Examples - 6
Problem #1 Solution (5/6)
ELEC1041 - Combinational Logic Examples - 7
Problem #1 Solution (6/6)
ELEC1041 - Combinational Logic Examples - 8
Problem #2
Consider a combinational logic subsystem that performs a two-bit
addition function. It has two 2-bit inputs A B and C D, and forms the
3-bit sum X Y Z.
(a) Draw the truth tables for XYZ(A,B,C,D).
(b)
Minimize the functions using 4-variable K-maps to derive
minimized Sum of Products forms.
(c)
In your textbook and in class/lab we have introduced the Full
Adder circuit. What is the relative performance to compute the
resulting sum bits of the 2-bit adder compared to two full adders
connected together? (Hint: which has the worst delay in terms of
gates to pass through between the inputs and the final outputs, and
how many gates is this?).
ELEC1041 - Combinational Logic Examples - 9
Problem #2 Solution (1/3)
(a) Draw the truth tables for XYZ(A,B,C,D).
0
1
2
3
ELEC1041 - Combinational Logic Examples - 10
00
01
10
11
Problem #2 Solution (2/3)
(b)
Minimize the functions using 4-variable K-maps to derive
minimized Sum of Products forms.
ELEC1041 - Combinational Logic Examples - 11
Problem #2 Solution (3/3)
(c)
In your textbook and in class/lab we have introduced the Full
Adder circuit. What is the relative performance to compute the
resulting sum bits of the 2-bit adder compared to two full adders
connected together? (Hint: which has the worst delay in terms of
gates to pass through between the inputs and the final outputs, and
how many gates is this?).
The above circuit is better because there are only two gate levels. But
the adder circuit can be used to build adder circuit of more bits. I
can’t imagine doing a K-map to construct a adder circuit of 64 bits
using the method above.
ELEC1041 - Combinational Logic Examples - 12
Problem #3
Show how to implement the full adder Sum(A, B, Cin) and Carry(A, B,
Cin) in terms of:
(a) Two 8:1 multiplexers;
(b) Two 4:1 multiplexers;
(c)
If you are limited to 2:1 multiplexers (and inverters) only, how
would you use them to implement the full adder and how many 2:1
multiplexers would you need?
ELEC1041 - Combinational Logic Examples - 13
Problem #3 Solution (1/8)
Ai
0
0
0
0
1
1
1
1
Bi
0
0
1
1
0
0
1
1
Cin
0
1
0
1
0
1
0
1
Sum
0
1
1
0
1
0
0
1
Cout
0
0
0
1
0
1
1
1
ELEC1041 - Combinational Logic Examples - 14
Problem #3 Solution (2/8)
(a) Two 8:1 multiplexers (Sum);
Ai
0
0
0
0
1
1
1
1
Bi
0
0
1
1
0
0
1
1
Cin
0
1
0
1
0
1
0
1
ELEC1041 - Combinational Logic Examples - 15
Sum
0
1
1
0
1
0
0
1
Problem #3 Solution (3/8)
(a) Two 8:1 multiplexers (Carry);
Ai
0
0
0
0
1
1
1
1
Bi
0
0
1
1
0
0
1
1
Cin
0
1
0
1
0
1
0
1
ELEC1041 - Combinational Logic Examples - 16
Cout
0
0
0
1
0
1
1
1
Problem #3 Solution (4/8)
(b) Two 4:1 multiplexers (Sum);
Ai
0
0
0
0
1
1
1
1
Bi
0
0
1
1
0
0
1
1
ELEC1041 - Combinational Logic Examples - 17
Cin
0
1
0
1
0
1
0
1
Sum
0
1
1
0
1
0
0
1
Problem #3 Solution (5/8)
(b) Two 4:1 multiplexers (Carry);
Ai
0
0
0
0
1
1
1
1
ELEC1041 - Combinational Logic Examples - 18
Bi
0
0
1
1
0
0
1
1
Cin
0
1
0
1
0
1
0
1
Cout
0
0
0
1
0
1
1
1
Problem #3 Solution (6/8)
(c) 2:1 multiplexers (sum)
Ai
0
0
0
0
1
1
1
1
ELEC1041 - Combinational Logic Examples - 19
Bi
0
0
1
1
0
0
1
1
Cin
0
1
0
1
0
1
0
1
Sum
0
1
1
0
1
0
0
1
Problem #3 Solution (7/8)
(c) 2:1 multiplexers (Carry)
Ai
0
0
0
0
1
1
1
1
ELEC1041 - Combinational Logic Examples - 20
Bi
0
0
1
1
0
0
1
1
Cin
0
1
0
1
0
1
0
1
Cout
0
0
0
1
0
1
1
1
Problem #3 Solution (8/8)
(c) 2:1 multiplexers (Sum) Ver 2 - USE the same carry
Ai
0
0
0
0
1
1
1
1
Bi
0
0
1
1
0
0
1
1
Cin
0
1
0
1
0
1
0
1
Sum
0
1
1
0
1
0
0
1
ELEC1041 - Combinational Logic Examples - 21
Problem #4
Design a combinational logic subsystem with three inputs, I3, I2, I1, and
two outputs, O1, O0, that behaves as follows. The outputs indicate
the highest index of the inputs that is driven high. For example, if I3
is 0, I2 is 1, I1 is 1, then O1, O0 would be 10 (i.e., I2 is the highest
input set to 1).
(a) Specify the function by filling out a complete truth table.
(b)
Develop the minimized gate-level implementation using the K-map
method.
(c) Develop an implementation using two 4:1 multiplexers.
(d)
Compare your implementation for (b) and (c). Which is better and
under what criterion?
ELEC1041 - Combinational Logic Examples - 22
Problem #4 Solution (1/5)
(a) Specify the function by filling out a complete truth table.
I3 I2
I1
O1
O0
0 0
0
0
0
0 0
1
0
1
0 1
x
1
0
1 x
x
1
1
ELEC1041 - Combinational Logic Examples - 23
Problem #4 Solution (2/5)
(b)
Develop the minimized gate-level implementation using the K-map
method.
I3 I2
I1
O1
O0
0 0
0
0
0
0 0
1
0
1
0 1
x
1
0
1 x
x
1
1
ELEC1041 - Combinational Logic Examples - 24
Problem #4 Solution (3/5)
(c) Develop an implementation using two 4:1 multiplexers.
I3 I2
I1
O1
O0
0 0
0
0
0
0 0
1
0
1
0 1
x
1
0
1 x
x
1
1
ELEC1041 - Combinational Logic Examples - 25
Problem #4 Solution (4/5)
(c) Develop an implementation using two 4:1 multiplexers.
I3 I2
I1
O1
O0
0 0
0
0
0
0 0
1
0
1
0 1
x
1
0
1 x
x
1
1
ELEC1041 - Combinational Logic Examples - 26
Problem #4 Solution (5/5)
(d)
Compare your implementation for (b) and (c). Which is better and
under what criterion?
ELEC1041 - Combinational Logic Examples - 27
Problem #5
You are to design a simple combinational subsystem to the following
specification. The system has the ability to pass its inputs directly to
its outputs when a control input, S, is not asserted. It interchanges
its inputs when the control input S is asserted. For example, given
four inputs A, B, C, D and four outputs W, X, Y, Z, when S=0,
WXYZ=ABCD and when S=1, WXYZ=BCDA.
Show how to implement this functionality using building blocks that are
restricted to be 2:1 multiplexers and 2:1 demultiplexers. Draw your
solution below, using black boxes for the mux/demux blocks.
A
B
C
D
W
X
Y
Z
S
ELEC1041 - Combinational Logic Examples - 28
Problem #5 Solution
Each output is tied to the output of a 2:1 Multiplexer. S is the select
signal.
A
W
B
X
C
Y
D
Z
S
ELEC1041 - Combinational Logic Examples - 29
Problem #6
Your task is to design a combinational logic subsystem to decode a
hexadecimal digit in the range of 0 through 9, A through F to drive a
seven-segment display. The hexadecimal numerals are as follows:
Design a minimized implementation in PLA form. That is, look for
common terms among the seven output functions.
ELEC1041 - Combinational Logic Examples - 30
Problem #6 Solution (1/5)
c1
c4
c0
c3
c2
c5
c6
c0 c1 c2 c3 c4 c5 c6
BCD to 7–segment
control signal
decoder
A B C D
ELEC1041 - Combinational Logic Examples - 31
Problem #6 Solution (2/5)
c1
c4
c0
c3
c2
c5
c6
c0 c1 c2 c3 c4 c5 c6
BCD to 7–segment
control signal
decoder
A B C D
ELEC1041 - Combinational Logic Examples - 32
A B C D
Problem #6 Solution (3/5)
BCD
BCD
ACD
BCD
ABD
BCD
ACD
ABC
ABC
ABD
ABC
ABC
ABC
ABD
ABD
ACD
ACD
ABC
F0 F1 F2 F3 F4 F5 F6
ELEC1041 - Combinational Logic Examples - 33
A B C D
Problem #6 Solution (4/5)
BCD
BCD
ACD
BCD
ABD
BCD
ACD
ABC
ABC
ABD
ABC
ABC
ABC
ABD
ABD
ACD
ACD
ABC
18 terms
F0 F1 F2 F3 F4 F5 F6
ELEC1041 - Combinational Logic Examples - 34
A B C D
Problem #6 Solution (5/5)
ABCD
ABCD
ABCD
ABCD
ABCD
ABCD
ABCD
ABCD
ABCD
ABCD
ABCD
ABCD
ABCD
ABCD
ABCD
ABCD
F0 F1 F2 F3 F4 F5 F6
ELEC1041 - Combinational Logic Examples - 35
16 terms
Problem #7
Calendar subsystem
Determine number of days in a month (to control watch display)
used in controlling the display of a wrist-watch LCD screen
inputs: month, leap year flag
outputs: number of days
ELEC1041 - Combinational Logic Examples - 36
Problem #7 Solution (1/3)
Calendar subsystem
Use software implementation to help understand the problem
integer number_of_days ( month, leap_year_flag) {
switch (month) {
case 1: return (31);
case 2: if (leap_year_flag == 1)
then return (29)
else return (28);
case 3: return (31);
case 4: return (30);
case 5: return (31);
case 6: return (30);
case 7: return (31);
case 8: return (31);
case 9: return (30);
case 10: return (31);
case 11: return (30);
case 12: return (31);
default: return (0);
}
}
ELEC1041 - Combinational Logic Examples - 37
Problem #7 Solution (2/3)
Formalize the problem Encoding:
binary number for month: 4 bits
4 wires for 28, 29, 30, and 31
one-hot – only one true at any time
Block diagram:
month
leap
28 29 30 31
month
0000
0001
0010
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
111–
ELEC1041 - Combinational Logic Examples - 38
leap
–
–
0
1
–
–
–
–
–
–
–
–
–
–
–
–
28
–
0
1
0
0
0
0
0
0
0
0
0
0
0
–
–
29
–
0
0
1
0
0
0
0
0
0
0
0
0
0
–
–
30
–
0
0
0
0
1
0
1
0
0
1
0
1
0
–
–
31
–
1
0
0
1
0
1
0
1
1
0
1
0
1
–
–
Problem #7 Solution (3/3)
Choose implementation target
and perform mapping Discrete gates
28 =
m8’ m4’ m2 m1’ leap’
29 =
m8’ m4’ m2 m1’ leap
30 =
m8’ m4 m1’ + m8 m1
31 =
m8’ m1 + m8 m1’
Can translate to S-o-P or P-o-S
month
0000
0001
0010
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
111–
leap
–
–
0
1
–
–
–
–
–
–
–
–
–
–
–
–
ELEC1041 - Combinational Logic Examples - 39
28
–
0
1
0
0
0
0
0
0
0
0
0
0
0
–
–
29
–
0
0
1
0
0
0
0
0
0
0
0
0
0
–
–
30
–
0
0
0
0
1
0
1
0
0
1
0
1
0
–
–
31
–
1
0
0
1
0
1
0
1
1
0
1
0
1
–
–
Problem #8
Leap year flag
Determine value of leap year flag given the year
For years after 1582 (Gregorian calendar reformation),
leap years are all the years divisible by 4,
except that years divisible by 100 are not leap years,
but years divisible by 400 are leap years.
ELEC1041 - Combinational Logic Examples - 40
Problem #8 Solution (1/5)
Leap year flag
Encoding the year:
binary – easy for divisible by 4, but difficult for 100 and 400 (not powers of 2)
BCD – easy for 100, but more difficult for 4, what about 400?
Parts:
construct a circuit that determines if the year is divisible by
construct a circuit that determines if the year is divisible by
construct a circuit that determines if the year is divisible by
combine the results of the previous three steps to yield the
ELEC1041 - Combinational Logic Examples - 41
4
100
400
leap year flag
Problem #8 Solution (2/5)
Divisible-by-4 circuit
BCD coded year
YM8 YM4 YM2 YM1 – YH8 YH4 YH2 YH1 – YT8 YT4 YT2 YT1 – YO8 YO4
YO2 YO1
Only need to look at low-order two digits of the year all years ending in 00, 04,
08, 12, 16, 20, etc. are divisible by 4 if tens digit is even, then divisible by 4
if ones digit is 0, 4, or 8 if tens digit is odd, then divisible by 4 if the ones
digit is 2 or 6.
Translates into the following Boolean expression (where YT1 is the year's tens
digit low-order bit, YO8 is the high-order bit of year's ones digit, etc.):
YT1’ (YO8’ YO4’ YO2’ YO1’ + YO8’ YO4 YO2’ YO1’ + YO8 YO4’ YO2’ YO1’ )
+ YT1 (YO8’ YO4’ YO2 YO1’ + YO8’ YO4 YO2 YO1’ )
Digits with values of 10 to 15 will never occur, simplify further to yield:
YT1’ YO2’ YO1’ + YT1 YO2 YO1’
ELEC1041 - Combinational Logic Examples - 42
Problem #8 Solution (3/5)
Divisible-by-100 and divisible-by-400 circuits
Divisible-by-100 just requires checking that all bits of two low-order digits are all
0:
YT8’ YT4’ YT2’ YT1’ • YO8’ YO4’ YO2’ YO1’
Divisible-by-400 combines the divisible-by-4 (applied to the thousands and
hundreds digits) and divisible-by-100 circuits
(YM1’ YH2’ YH1’ + YM1 YH2 YH1’) • (YT8’ YT4’ YT2’ YT1’ • YO8’ YO4’ YO2’ YO1’
)
ELEC1041 - Combinational Logic Examples - 43
Problem #8 Solution (4/5)
Combining to determine leap year flag
Label results of previous three circuits: D4, D100, and D400
leap_year_flag
= D4 (D100 • D400’ ) ’
= D4 • D100’ + D4 • D400
= D4 • D100’ + D400
ELEC1041 - Combinational Logic Examples - 44
Problem #8 Solution (5/5)
Implementation of leap year flag
ELEC1041 - Combinational Logic Examples - 45
Problem #9
Consider the following functions, which are five different functions over the
inputs A,B,C,D.
(1) F(A,B,C) = m(1,2,6,7)
(2)
(3)
(4)
(5)
F(A,B,C,D) = m(0,1,3,9,11,12,14,15)
F’(A,B,C,D) = m(2,4,5,6,7,8,10,13)
F(A,B,C,D) = (A B C + A’ B’) (C + D)
F(A,B,C,D) = (A + B + C) (A + B + C’ + D) (A + B’ + C + D’) (A’ + B’)
a) you were to implement these in a single PLA structure with four inputs, five
outputs, and an unlimited number of product terms, how many unique
product terms are there in this PLA implementation
b) If you are trying to maximize the number of shared product terms across the
five functions, rather than minimizing the literal count for each function
independently, how many unique terms do you obtain? Draw the new Kmaps with your selection of implicants that minimizes the number of unique
terms across all five functions
ELEC1041 - Combinational Logic Examples - 46
Problem #9 Solution (1/2)
Consider the following functions, which are five different functions over the
inputs A,B,C,D.
(1) F(A,B,C) = m(1,2,6,7)
(2) F(A,B,C,D) = m(0,1,3,9,11,12,14,15)
(3) F’(A,B,C,D) = m(2,4,5,6,7,8,10,13)
(4) F(A,B,C,D) = (A B C + A’ B’) (C + D)
(5)
F(A,B,C,D) = (A’ + B + C) (A + B + C’ + D) (A + B’ + C + D’) (A’ + B’)
a) you were to implement these in a single PLA structure with four inputs,
five outputs, and an unlimited number of product terms, how many unique
product terms are there in this PLA implementation
F(A,B,C)= BC’+AB+A’B’C
F(A,B,C,D)=ABD’+ACD+B’D+A’B’C’
F’(A,B,C,D)=A’B+BC’D+AB’D’+B’CD’
F(A,B,C,D)=A’B’D+A’B’C+ABC
F(A,B,C,D)=A’C’D’+A’B’D+A’BC+AB’C
From Revision Tutorial
17 unique terms
ELEC1041 - Combinational Logic Examples - 47
Problem #9 Solution (2/2)
b) If you are trying to maximize the number of shared product terms across the five functions,
rather than minimizing the literal count for each function independently, how many unique
terms do you obtain? Draw the new K-maps with your selection of implicants that minimizes
the number of unique terms across all five functions
1. BC’ + AB + A’B’C
=> ABD' + ABD + A'BC' + ABC' + A'B'C
2. ABD’ + ACD + B’D + A’B’C’ => ABD' + ACD + AB'D + A'B'D + B'CD + B'C'D + A'B'C'
3. A’B + BC’D + AB’D’ + B’CD’ => A'BC + A'BC' + A'BD + A'BD' + BC'D + AB'D' + B'CD'
4. A’B’D + A’B’C + ABC
=> A’B’D + A’B’C + ABC
5. A’C’D’+A’B’D+A’BC+AB’C
=> A’C’D’+A’B’D+A’BC+AB’C
shared terms:
A'B'C is shared between 1 and 4.
A’BC’ is shared between 1 and 3.
ABD' is shared between 1 and 2.
A’B’D is shared between 2, 4 and 5.
A’BC is shared between 3 and 5.
5 shared terms +[22 – (4  2) – (1  3)] => 11 unique terms across all five functions
NOTE: PLA Minimization is always carried out by CAD Tools
ELEC1041 - Combinational Logic Examples - 48
Problem #10
Consider the following Boolean function in Product of Sums form:
F(A,B,C,D) = (A + B’ + D) (A’ + B’ + D) (B’ + C’ + D’) (A’ + C + D) (A’ + C’ + D)
Show how to implement this function with an 8:1 multiplexer, with A,B,C on the
control inputs and D, its complement, and the constants 0 and 1 available as data
inputs. Show your work:
0
1
2
3
F(A,B,C,D)
4
5
6
7
S2 S1 S0
A B C
ELEC1041 - Combinational Logic Examples - 49
Problem #10 Solution (1/2)
F(A,B,C,D) = (A + B’ + D) (A’ + B’ + D) (B’ + C’ + D’) (A’ + C + D) (A’ + C’ + D)
0
1
2
3
F(A,B,C,D)
4
5
6
7
S2 S1 S0
A B C
ELEC1041 - Combinational Logic Examples - 50
Problem #10 Solution (2/2)
F(A,B,C,D) = (A + B’ + D) (A’ + B’ + D) (B’ + C’ + D’) (A’ + C + D) (A’ + C’ + D)
1
1
D
0
D
D
D
)
0
1
2
3
4
5
6
7
F(A,B,C,D)
S2 S1 S0
A B C
ELEC1041 - Combinational Logic Examples - 51
Problem #11
Consider a variation on the calendar combinational subsystem that works
as follows. Given the inputs MONTH (1-12), DAY (1-31), and LEAP_YEAR
flag, the subsystem generates the output DAY_OF_YEAR (1-365 or 366). In
this problem, you will design the subsystem to the block diagram level only.
(a)
One block maps the month into a day offset into the year. Identify the
inputs and outputs and their bit widths. Use any formal specification method
you wish (e.g., truth tables, ROM contents, equations, hardware description
language, etc.) to describe the function of this block.
(b)
You may assume any width binary adder you may require. Indicate how
the adder is composed with the block of part (a) and any other blocks or
inputs to compute the correct output. Be sure to describe how you deal with
the LEAP_YEAR input
ELEC1041 - Combinational Logic Examples - 52
Problem #11 Solution (1/3)
l
(a) One block maps the month into a day offset into the year. Identify the inputs and outputs and their
bit widths. Use any formal specification method you wish (e.g., truth tables, ROM contents, equations,
hardware description language, etc.) to describe the function of this block
Day
m4 m3 m2 m1 m0
d8 d7 d6 d5 d4 d3 d2 d1 d0
Offset
x
0
0
0
0
1
0
0
0
0
0
0
0
0
0
0
x
0
0
0
1
0
0
0
0
0
1
1
1
1
1
31
0
0
0
0
1
1
0
0
0
1
1
1
0
1
1
59
1
0
0
0
1
1
0
0
0
1
1
1
1
0
0
60
0
0
0
1
0
0
0
0
1
0
1
1
0
1
0
90
1
0
0
1
0
0
0
0
1
0
1
1
0
1
1
91
-----
ELEC1041 - Combinational Logic Examples - 53
Problem #11 Solution (2/3)
(a) One block maps the month into a day offset into the year. Identify the inputs and outputs and their
bit widths. Use any formal specification method you wish (e.g., truth tables, ROM contents, equations,
hardware description language, etc.) to describe the function of this block
m4
m3
m2
m1
m0
l
ROM
26 x 10
d8
d7
d6
d5
d4
d3
d2
d1
d0
ELEC1041 - Combinational Logic Examples - 54
Problem #11 Solution (3/3)
(b)
You may assume any width binary adder you may require. Indicate
how the adder is composed with the block of part (a) and any other blocks or
inputs to compute the correct output. Be sure to describe how you deal with
the LEAP_YEAR input
Since leap has been taken care of in part a, you can just add whatever output
from that block with day, which will give you the correct day of the year.
m4
m3
m2
m1
m0
l
ROM
26 x 10
day8 – day5
are always 0
day8
day7
day6
day5
day4
day3
day2
day1
day0
d8
d7
d6
d5
d4
d3
d2
d1
d0
d8-d0
day8 – day0
ELEC1041 - Combinational Logic Examples - 55
9-bit
Adder
d8
d7
d6
d5
d4
d3
d2
d1
d0
Problem #12
Design a two-bit comparator with the following inputs and outputs:
Inputs: Numbers N1 and N2 to be compared
N1 = A B
N2 = C D
Outputs: LT, GT, EQ
LT = 1 when AB < CD
example (A B = 01) < (C D = 10)
GT = 1 when AB > CD
example (A B = 11) > (C D = 10)
EQ = 1 when AB = CD
example (A B = 01) = (C D = 01)
ELEC1041 - Combinational Logic Examples - 56
Problem #12 Solution (1/3)
A B
0 0
N1
N2
A
B
C
D
LT
EQ
GT
AB<CD
AB=CD
AB>CD
block diagram
and
truth table
0
1
1
0
1
1
C
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
D
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
LT
0
1
1
1
0
0
1
1
0
0
0
1
0
0
0
0
EQ
1
0
0
0
0
1
0
0
0
0
1
0
0
0
0
1
GT
0
0
0
0
1
0
0
0
1
1
0
0
1
1
1
0
we'll need a 4-variable Karnaugh map
for each of the 3 output functions
ELEC1041 - Combinational Logic Examples - 57
Problem #12 Solution (2/3)
A
C
A
0
0
0
0
1
0
0
0
1
1
0
1
1
1
0
0
D
C
A
1
0
0
0
0
1
0
0
0
0
1
0
0
0
0
1
D
C
0
1
1
1
0
0
1
1
0
0
0
0
0
0
1
0
B
B
B
K-map for LT
K-map for EQ
K-map for GT
D
LT = A' B' D + A' C + B' C D
EQ = A'B'C'D' + A'BC'D + ABCD + AB'CD’
= (A xnor C) • (B xnor D)
GT = B C' D' + A C' + A B D'
LT and GT are similar (flip A/C and B/D)
ELEC1041 - Combinational Logic Examples - 58
Problem #12 Solution (3/3)
A
B C
D
two alternative
implementations of EQ
with and without XOR
EQ
EQ
XNOR is implemented with
at least 3 simple gates
ELEC1041 - Combinational Logic Examples - 59
Problem #13
Design a 2X2 bit multiplier:
Inputs: Numbers N1 and N2 to be multiplied
N1 = A1 A0
N2 = B1 B0
Outputs: products: P8, P4, P2, P0
P0 = Product with weighting 20 = 1
P2 = Product with weighting 21 = 2
P4 = Product with weighting 22 = 4
P8 = Product with weighting 23 = 8
Example:
11
11
11
11
1001
P8 P4 P2 P0
ELEC1041 - Combinational Logic Examples - 60
A1A0
B1B0
B0(AA0)
B1(AA0)
Problem #13 Solution (1/2)
A1
A2
B1
B2
P1
P2
P4
P8
block diagram
and
truth table
A2 A1 B2
0 0 0
0
1
1
0 1 0
0
1
1
1 0 0
0
1
1
1 1 0
0
1
1
B1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
P8
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
P4
0
0
0
0
0
0
0
0
0
0
1
1
0
0
1
0
P2
0
0
0
0
0
0
1
1
0
1
0
1
0
1
1
0
4-variable K-map
for each of the 4
output functions
ELEC1041 - Combinational Logic Examples - 61
P1
0
0
0
0
0
1
0
1
0
0
0
0
0
1
0
1
Problem #13 Solution (2/2)
A2
B2
0
0
0
0
0
0
0
0
0
0
1
0
0
0
0
0
K-map for P8
B1
K-map for P4
P4 = A2B2B1'
+ A2A1'B2
P8 = A2A1B2B1
B2
A2
0
0
0
0
0
0
0
0
0
0
0
1
0
0
1
1
A1
A1
A2
B2
0
0
0
0
0
0
1
1
0
1
0
1
0
1
1
0
A1
B1
K-map for P2
K-map for P1
P1
= A1B1
B1
P2 = A2'A1B2
+ A1B2B1'
+ A2B2'B1
+ A2A1'B1
B2
ELEC1041 - Combinational Logic Examples - 62
A2
0
0
0
0
0
1
1
0
0
1
1
0
0
0
0
0
A1
B1
Problem #14
Design a BCD increment by 1:
Inputs: BCD Number N
N = I8 I4 I2 I0
Outputs: products: O8, O4, O2, O0
O8 O4 O2 O0 = I8 I4 I2 I0 + 1
Example:
0011
0100
O8 O4 O2 O0
ELEC1041 - Combinational Logic Examples - 63
I8 I4 I2 I0
O8 O4 O2 O0
Problem #14 Solution (1/2)
I1
I2
I4
I8
O1
O2
O4
O8
block diagram
and
truth table
I8
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
I4
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
I2
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
I1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
O8
0
0
0
0
0
0
0
1
1
0
X
X
X
X
X
X
O4
0
0
0
1
1
1
1
0
0
0
X
X
X
X
X
X
O2
0
1
1
0
0
1
1
0
0
0
X
X
X
X
X
X
4-variable K-map for each of
the 4 output functions
ELEC1041 - Combinational Logic Examples - 64
O1
1
0
1
0
1
0
1
0
1
0
X
X
X
X
X
X
Problem #14 Solution (2/2)
I8
I2
0
0
X
1
0
0
X
0
0
1
X
X
0
0
X
X
O8
I1
I2
0
0
X
0
1
1
X
0
0
0
X
X
1
1
X
X
O8 = I4 I2 I1 + I8 I1'
O4 = I4 I2' + I4 I1' + I4’ I2 I1I2
O2 = I8’ I2’ I1 + I2 I1'
I4
I8
O4
O2
O1 = I1'
0
1
X
0
0
1
X
0
1
0
X
X
0
1
X
X
O1
I2
I8
1
1
X
1
0
0
X
0
0
0
X
X
1
1
X
X
I4
ELEC1041 - Combinational Logic Examples - 65
I1
I4
I1
I4
I8
I1
Problem #15
Analyse the behavior of the Circuit below when Input A changes from one logic
state to another.
A
B
C
D
F
ELEC1041 - Combinational Logic Examples - 66
Problem #15 Solutions (1/1)
 pulse shaping circuit (Momentary Changes in outputs)
 A' xnor A = 0
 delays matter in function
1
0A
d
0
d
1B
d
1
0
0
1D
C
0
0
d
1
F
1
D remains high for
three gate delays after
A changes from low to high
F is not always 0
pulse 3 gate-delays wide
D remains low for
three gate delays after
A changes from high to low
ELEC1041 - Combinational Logic Examples - 67
3d
0
Problem #16
Analyse the circuit below for static hazard
A
S
B
S'
ELEC1041 - Combinational Logic Examples - 68
Problem #16 Solution (1/1)
 Due to a literal and its complement momentarily taking on the same value
through different paths with different delays and reconverging we get Static
1 Hazard
A
S
1
1
1
0
B
F
0
0
B
d
A
1
d
1
2d
0
1
1
2d
S
0
3d
1 S'
F
S'
Static-1 hazard
static-0 hazard
static-1 hazard
ELEC1041 - Combinational Logic Examples - 69
Problem #17
Analyse the pulse shaping circuit below
+
open
switch
A
B
C
D
ELEC1041 - Combinational Logic Examples - 70
Problem #17 Solution (1/1)
 An Oscillator
+
open
switch
U = Undefined
A
0
1
C
U
1
2d
close switch
initially
undefined open switch
ELEC1041 - Combinational Logic Examples - 71
BU d 0
D
U
3d
0
Problem #18
Which of the components below cab be used to build an inverter?
ELEC1041 - Combinational Logic Examples - 72
Problem #18 Solution (1/1)
1
A’
A
A nand 1 = A’
0
A’
A
A nor 0 = A’
1
A’
A
A xor 1 = A’
0
A’
A
A xnor 0 = A’
ELEC1041 - Combinational Logic Examples - 73
Problem #19
Consider the Equation:
Z= A’ B’ C’ D + A’ B’ C D’ + A’ B C’ D’ + A’ B C D + A B C’ D + A B C D’
+ A B’ C’ D’ + A B’ C D
Use Shanon’s decomposition to implement this using 2-1 multiplexers.
ELEC1041 - Combinational Logic Examples - 74
Problem #19 Solution (1/3)
Z= A’ B’ C’ D + A’ B’ C D’ + A’ B C’ D’ + A’ B C D + A B C’ D + A B C D’
+ A B’ C’ D’ + A B’ C D
Z = A’ [B (C’ D’ + C D) + B’ (C’ D + C D’)] + A [B (C’ D + C D’) + B’ (C’
D’ + C D)]
= A’ [B (C xor D)’ + B’ (C xor D)] + A [B (C xor D) + B’ (C xor D)’]
= A’ [( B xor C xor D)] + A [( B xor C xor D)’]
= A xor B xor C xor D
ELEC1041 - Combinational Logic Examples - 75
Problem #19 Solution (2/3)
Z= A xor B xor C xor D
Decompose around A:
Z = A’ (B xor C xor D) + A (B xor C xor D)’
( B xor C xor D)
Z
Now Decompose (B xor C xor D) around B:
B xor C xor D = B’ (C xor D) + B (C xor D)’
Z
C xor D
ELEC1041 - Combinational Logic Examples - 76
Problem #19 Solution (3/3)
Now Decompose (C xor D) around C:
C xor D = C’ D + B D’
D
Z
ELEC1041 - Combinational Logic Examples - 77
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