Software Developers View of Hardware

Electronic Circuits

What are circuits?

Computers are electrical devices, so therefore all functions performed by a computer need to done via the use of circuits.

Circuits are designed via the use of Logic Gates which show the path and the way in which electronic signals are sent and received.

Logic Gates

Are a hardware circuit that produces a 0 or 1, which is normally an electronic impulse.

There are THREE basic logic gates and THREE extended gates that can be used to build integrated circuits.

BASIC GATES

1. NOT Gate This is the simplest of all gates, it involves a

single input and a single output. The purpose of this gate is the flipping of a bit

similar to what is performed in one’s complement.

NOT Gate

0 1A X

NOT Gate

NOT Gate

NOT Gate – Truth Table

A X

0 1

1 0

IF A = 0 THEN

X = 1

ELSE

X = 0

ENDIF

BASIC GATES

2. AND Gate This is involves two inputs to produce one

output. Both inputs must be true for the output to be true.

AND Gate

A

X

B

AND Gate

AND Gate

AND Gate

AND Gate – Truth Table

A B X

0 0 0

0 1 0

1 0 0

1 1 1

IF A=1 AND B=1THEN

X = 1

ELSE

X = 0

ENDIF

BASIC GATES

3. OR Gate This is involves two inputs to produce one

output. If either inputs are true then the output will be

true.

OR Gate

A

X

B

OR Gate

OR Gate

OR Gate

OR Gate

OR Gate – Truth Table

A B X

0 0 0

0 1 1

1 0 1

1 1 1

IF A=1 OR B=1THEN

X = 1

ELSE

X = 0

ENDIF

Activity 1

Complete the truth table for the following circuit.

A

B

YX

Truth Table

A B X Y

0 0

0 1

1 0

1 1

EXTENDED GATES

1. NAND Gate This is involves two inputs to produce one

output. The output is the opposite of an AND gate. Is a combination of an AND and NOT gate.

NAND Gate

A

X

B

NAND Gate – Truth Table

A B X

0 0 1

0 1 1

1 0 1

1 1 0

IF A=1 AND B=1THEN

X = 0

ELSE

X = 1

ENDIF

EXTENDED GATES

2. NOR Gate This is involves two inputs to produce one

output. The output is the opposite of an OR gate. It is a combination of an OR and NOT.

NOR Gate

A

X

B

NOR Gate – Truth Table

A B X

0 0 1

0 1 0

1 0 0

1 1 0

IF A=1 AND B=1THEN

X = 0

ELSE

X = 1

ENDIF

EXTENDED GATES

3. XOR Gate This stands for exclusive OR. This gate is true if only one input is true.

XOR Gate

A

X

B

XOR Gate – Truth Table

A B X

0 0 0

0 1 1

1 0 1

1 1 0

SPECIALITY CIRCUITS

Designed to make use of our binary knowledge and our circuitry knowledge

Examples include: Adders Flip Flops Shifts

DESIGNING SPECIALITY CIRCUITS

These circuits are written to provide a specific function: Adder (Binary Addition) Flip Flop (Binary Storage)

DESIGNING SPECIALITY CIRCUITS

Follow these steps: Identify inputs and outputs Identify the components required to produce the

output (AND, OR, NOT, NAND, NOR, XOR) Construct the solution with logic gates Check the solution for validity (with a truth table) Evaluate the circuit design (could you make this

circuit better by chaining different logic gates)

Binary Half Adder

This device is basically a calculator. Lets look at the half adder truth table first.

Binary Half Adders

To create a Binary Adder, we need to find a logic gate that give us the Carry output and a logic gate the Sum output

INPUT OUTPUT

A B Carry Sum

0 0 0 0

0 1 0 1

1 0 0 1

1 1 1 0

Binary Half Adders

Carry output is created using a

INPUT OUTPUT

A B Carry Sum

0 0 0 0

0 1 0 1

1 0 0 1

1 1 1 0

AND logic gate

AX

B

Binary Half Adders

Sum output is created using a

INPUT OUTPUT

A B Carry Sum

0 0 0 0

0 1 0 1

1 0 0 1

1 1 1 0

XOR logic gate

AX

B

The circuit:

Binary Half Adders

A

B

Carry (C)

Sum (S)

Half And Full Adders

Half Adders only work to add two digits To add more than 2 binary digits we need a full

adder A full adder allows us to add the “carry” value to

an binary addition

Full Adders

A

B

Carry (C)

Sum (S)

Carry in

Truth Tables

Construct a truth table for the full adder.

Truth Table

A B CARRY IN

CARRY SUM

0 0 0 0 0

0 0 1 0 1

0 1 0 0 1

0 1 1 1 0

1 0 0 0 1

1 0 1 1 0

1 1 0 1 0

1 1 1 1 1

Circuit Design Steps

Identify inputs and outputs.

A + B + C = X Identify the components needed to obtain

the desired output.

AND/OR/NOT/XOR/NAND/NOR Construct a truth table to test.

Activity 2

Construct a truth table for the following circuit.

YAB X

C

A B C Y X0 0 0 1 10 0 1 1 00 1 0 1 10 1 1 1 01 0 0 1 11 0 1 1 01 1 0 0 01 1 1 0 1

AB X

C

Activity 3

Fault Door Switch x Light0 0 0 00 0 1 00 1 0 00 1 1 11 0 0 01 0 1 01 1 0 01 1 1 0