A Deep Dive on Digital Systems: Exploring Topics within the FE Electrical Exam
Mar 08, 2024For those looking to take the Fundamentals of Engineering (FE) Electrical Engineering (EE) Exam we will delve into various concepts and components of digital systems. The topics can be found on this link.They include:
- Number systems
- Boolean logic
- Logic gates and circuits
- Logic minimization (e.g., Sum of Product (SOP), Product of Sum (POS), Karnaugh maps)
- Flip-flops and counters
- Programmable logic devices and gate arrays
- State machine design
- Timing (e.g., diagrams, asynchronous inputs, race conditions, and other hazards)
Please note that while we aim to cover a wide range of information, we have limited space and time. Therefore, we encourage you to explore further resources if you wish to delve deeper into any particular topic.
Number System
The number system refers to the different ways in which numerical values can be represented and manipulated. This is done to simplify and compress large numerical values into a few easily interpreted digits. Digital systems rely on these number systems: decimal system, binary system, hexadecimal system, and octal system.
- Decimal System
The decimal system consists of a set of 10 symbols or numerals, namely 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. By utilizing these symbols as the digits of a number, we can represent any quantity within the decimal system (Widmer et al., 2017, 19). - Binary System
In the binary system, we only have 0 and 1, and each binary digit has its own value expressed as a power of 2 (Widmer et al., 2017, 21). The concept is depicted in Figure 1, where positions to the left of the binary point, analogous to the decimal point, represent positive powers of 2, while positions to the right represent negative powers of 2.
Figure 1 - Hexadecimal System
Hexadecimal is a numbering system represented in base 16, utilizing both numbers "0-9" and the alphabet "A-F" for double digit values "10-15". - Octal System
This numerical system is a base-8 system, using digits "0-7"
Boolean Logic
Boolean logic, named after mathematician and logician George Boole, is a fundamental concept in digital systems and computer science. It is a branch of mathematics and logic that deals with variables and operations based on the principles of true and false, or 1 and 0, respectively. Boolean algebra, unlike other mathematical systems, does not involve fractions, decimals, negative numbers, square roots, cube roots, logarithms, imaginary numbers, and similar mathematical concepts (Widmer et al., 2017, 71). Table 1 shows the common logic terms for the 0 and 1.
|
Logic
0 |
Logic 1 |
|
False |
True |
|
Off |
On |
|
Low |
High |
|
No |
Yes |
|
Open switch |
Closed switch |
Logic Gates
The three basic operations are NOT, AND (*), and OR (+). Below, you will find the definition, logic symbol, and corresponding Boolean expression for each function.
1. NOT Operation
Figure x
The NOT operator or inverter (Figure x) inverts the sense of a binary value (0→1, 1→0) . It can be referred to as inversion or complementation. The NOT operation can be expressed as (Widmer et al., 2017, 80):
x = A or x = A'
The truth table is depicted below.
|
A |
x |
|
0 |
1 |
|
1 |
0 |
2. AND Operation
Figure x1
To illustrate the application of AND logic, let's take the example of a standard clothes dryer. The dryer will perform the drying process, including heating and tumbling, only if two conditions are met simultaneously: the timer is set above zero AND the door is closed. The AND circuit is illustrated in Figure x1. The AND operation can be expressed as (Widmer et al.,
2017, 77-78):
x = A . B
The truth table is depicted below.
|
A |
B |
x = A . B |
|
0 |
0 |
0 |
|
0 |
1 |
0 |
|
1 |
0 |
0 |
|
1 |
1 |
1 |
3. OR Operation
Figure x2
Let's consider an example in a kitchen oven. The scenario involves the oven's light, which should be activated under two conditions: either the oven light switch is in the "on" position OR the oven door is opened. To represent these conditions, we can use variables A to denote the state of the oven light switch (true or false) and B to represent the state of the oven door (true or false). The variable x can then represent the state of the light (true or false). The AND circuit is illustrated in Figure x2. The OR operation can be expressed as(Widmer et al., 2017, 73-74):
x = A + B
The truth table is depicted below.
|
A |
B |
x = A + B |
|
0 |
0 |
0 |
|
0 |
1 |
1 |
|
1 |
0 |
1 |
|
1 |
1 |
1 |
Logic Minimization
Logic minimization refers to the process of
simplifying and optimizing logical expressions or equations. It involves
reducing the complexity of logical functions by eliminating redundancies and
minimizing the number of logic gates required to implement the function.
There are two simplifications we will explore:
sum-of-products (SOP) and product-of-sums (POS).
1. Sum-of-Products
1. Sum-of-Products
SOP consists of two
or more AND terms that are ORed together. SOP can also be referred to as
Minterm. The function used to denote the sum of minterms is expressed below
(Widmer et al., 2017, 138).
The SOP is selected by f (x ,y ,z) = 1
2. Product-of-SumsPOS consists of two
or more OR terms that are ANDed together. POS can also be referred to as
Maxterm. The function used to denote the product of maxterms is expressed below
(Widmer et al., 2017, 139).
The POS is selected by F (x ,y ,z) = 0
Given the truth table below, the expression of the SOP-Minterms and POS-Maxterms for the three binary variables can be obtained.
|
A |
B |
C |
Minterms |
Maxterms |
||
|
Term |
Designation |
Term |
Designation |
|||
|
0 |
0 |
0 |
A'B'C' |
m0 |
A+B+C |
M0 |
|
0 |
0 |
1 |
A'B'C |
m1 |
A+B+C' |
M1 |
|
0 |
1 |
0 |
A'BC' |
m2 |
A+B'+C' |
M2 |
|
0 |
1 |
1 |
A'BC |
m3 |
A+B'+C' |
M3 |
|
1 |
0 |
0 |
AB'C' |
m4 |
A'+B+C |
M4 |
|
1 |
0 |
1 |
AB'C |
m5 |
A'+B+C' |
M5 |
|
1 |
1 |
0 |
ABC' |
m6 |
A'+B'+C |
M6 |
|
1 |
1 |
1 |
ABC |
m7 |
A'+B'+C |
M7 |
Example: Find the expression for the function f as (a) sum of minterms and (b) product of maxterms using the following truth table.
|
A |
B |
C | f |
|
0 |
0 |
0 |
0 |
|
0 |
0 |
1 |
1 |
|
0 |
1 |
0 |
0 |
|
0 |
1 |
1 |
0 |
|
1 |
0 |
0 |
1 |
|
1 |
0 |
1 |
0 |
|
1 |
1 |
0 |
0 |
|
1 |
1 |
1 |
1 |
a) By following the rule of SOP, the sum of minterms can be obtained
f = A'B'C + AB'C' + ABC = m1 + m4 + m7
b) By following the rule of POS, the product of maxterms can be obtained
f = (A+B+C)(A+B'+C)(A+B'+C')(A'+B+C')(A'+B'+C) = M0 M2 M3 M5 M6
3. Karnaugh Map
A Karnaugh map consists of a grid-like structure, with input variables represented along the axes. Each cell in the grid corresponds to a specific combination of input variable values. The cells are typically labeled with binary values or decimal numbers, depending on the complexity of the function.
To simplify a logical function using a Karnaugh map, adjacent cells with 1s are identified and grouped together. These groupings, known as minterms, represent areas of the map where the logical function evaluates to true (1). By combining adjacent cells, redundant terms and logic gates can be eliminated, resulting in a simplified expression (Widmer et al., 2017, 152). Below are the steps on how to use Karnaugh maps or K-map to simplify logic expression (Widmer et al., 2017, 157-158).
a. Create a table
● In the FE Reference Handbook, you can find the K-map format in page xx to set up the table.
Figure x: K-map for 3 variables
Figure x: K-map for 4 variables
b. Complete the K-map from the truth table
● Place ‘0’ or ‘1’ into the corresponding cell term
|
A |
B |
C |
f |
|
0 |
0 |
0 |
1 |
|
0 |
0 |
1 |
1 |
|
0 |
1 |
0 |
1 |
|
0 |
1 |
1 |
0 |
|
1 |
0 |
0 |
0 |
|
1 |
0 |
1 |
0 |
|
1 |
1 |
0 |
1 |
|
1 |
1 |
1 |
0 |
Figure x
c. Form group of 1s
● Identify the 1s that are adjacent to only one other 1. Loop any pair containing such a 1.
● Groups can wrap around the edges and corners of the K-map.
● The “1” cells must be loop in an even group (groups of two, group of four, or group of eight). Make sure to use the minimum number of loops.
d. Form the Boolean expression
○ Each group represents the SOP equation.
f = A'B + BC'
The red group shares A = 0 and B = 0.
The blue group shares B = 1 and C = 0
e. The “Don’t Care”
○ “Don’t Care” is illustrated as an X in the truth table. It can be used as a ‘0’ or a ‘1’, whatever makes the looping easier.
f = A' + C'D + BD
Conclusion Part 1
This is a lot of information to sort through on your first reading; please take the time to reread and practice problems on your own. We will finish the other half of these topics in an upcoming blog. Again, as this is only an introduction to these topics, it is highly encouraged that examinees explore School of PE’s FE Electrical exam review course to complement their exam prep.
References
Widmer, N. S., Tocci, R. J., & Moss, G. L. (2017). Digital Systems: Principles and Applications. Pearson.
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