Topic outline
 General
 Topic 1: Introduction to Digital Electronics (Seminar 1 Friday 14th December)
Topic 1: Introduction to Digital Electronics (Seminar 1 Friday 14th December)
In this section, you will understand key concepts on digital electronics and work through relevant exercises.
Why Study Digital Electronics?

Electronic devices can be either analogue or digital.

Most devices incorporate some of both types but digital electronics is used to greater and greater extent because it is always getting faster and cheaper.

All computers use digital electronic circuits.

Everything on the internet is digital.

Modern mobile phones are mostly digital, as are televisions, MP£ players, cameras, etc.

What Makes a circuit “Digital”?

All digital devices are made from circuits which can be switched between two possible states.

These two states are represented by a voltage level at the output stage of the circuit. For example, 0V and 5V.

For convenience these two states are called logic level 0 or ‘low’ for the lower voltage and logic level 1 or ‘high’ for the higher voltage.
 Modern mobile phones are mostly digital, as are
televisions, MP£ players, cameras, etc.

What Can Digital Circuits Do?

Circuits can be designed to implement a specific task. For example, a simple circuit could compare two input voltages and give a high output if they matched and a low if they did not match – a ‘comparator’.

More complex circuits can be designed to make such devices such as MP3 players, computers, etc.

Manipulating Numbers

Computers manipulate numbers – but decimal numbers with digits 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9 can not be represented using two states of a digital circuit.

Instead decimal numbers are converted to the equivalent binary numbers.
• Binary numbers have only two digits 0 and 1 and these can be represented using two states.

Decimal and Binary Numbers
Decimal 0
Binary 0
1
0
2
01
3
11
4
100
5
101
6
110
7
111
8
1000
9
1001
Manipulating Logical Expressions

Digital circuits also manipulate logical expression. For example, IF account is in credit THEN allow phone to make a call.

So a digital circuit must determine if something is TRUE or FALSE. Normally logic level 1 is used to represent TRUE and logic level 0 represent FALSE.

Bit it could be 0 for TRUE and 1 for FALSE. There are two alternative ‘coding's’.

Examples of coding
• Examples of coding might be for the condition of a light switch with on coded as 1 and off coded as 0.
• Another example is the direction of a compass:
– Compass Point: Coding – North 00 – East 01 – south
10 – west 11 Combinational Logic
• At a particular moment in time the output of a COMBINATIONAL logic circuit depends upon the inputs to the circuit at the same instant.
• Unlike sequential logic where the output may also depend upon the past inputs.
• The state of the output for each COMBINATION of the inputs determines the function of the circuit.
Combinational Logic Circuits
• A circuit can be designed to perform many different functions. For example;
• A circuit has 3 inputs A, B, and C and 3 outputs X, Y and Z:
– Output X is logic level 1 (or ‘high’) if one or more inputs are at logic level 1 (or ‘high’).
– Output Y is 1 if two of more of the inputs are at 1. – Output Z is 1 if all three inputs are at logic level 1.
Basic Logic Gates
• The following slides will introduce the basic logic gates in terms of:
– Their function

– Their circuit Symbol

– Their truth table

– Their equivalent in Boolean algebra (a mathematical method based on human reasoning which will be explained later)

The NOT Gate or Inverter (I)
• The NOT gate is the simplest of all logic circuits. It has just one input and one output.
• If the input is a logic level 1 then the output is logic level 0.
• Similarly if the input is 0 then the output is 1 so that the output is the inverse of the input.
The NOT Gate or Inverter (I)
• The NOT gate is the simplest of all logic circuits. It has just one input and one output.
• If the input is a logic level 1 then the output is logic level 0.
• Similarly if the input is 0 then the output is 1 so that the output is the inverse of the input.
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