Module 7

Hardware

Introduction

• Switches are the basic blocks of computer hardware.

• We build increasingly complex hardware from these simple switches.

• An example of a switch that we use everyday in our lives is the Light Switch.

Introduction

• The switches that we use in computer hardware are far too small to be seen by the naked eye.

• Then how are these components constructed if they are that small?

Introduction

• The answer rests in three technologies:

Introduction

• The answer rests in three technologies:– The idea of representing information by electrical

signals which led to development of the telegraph. • Switching devices developed such as Vacuum tubes,

but they all suffered from the disadvantages of being large, high power consumption and slow speed.

Introduction

• The answer rests in three technologies:– The idea of representing information by electrical

signals which led to development of the telegraph. • Switching devices developed such as Vacuum tubes,

but they all suffered from the disadvantages of being large, high power consumption and slow speed.

– Invention of transistor which is a very small switch.

Introduction• The answer rests in three technologies:– The idea of representing information by electrical

signals which led to development of the telegraph. • Switching devices developed such as Vacuum tubes, but

they all suffered from the disadvantages of being large, high power consumption and slow speed.

– Invention of transistor which is a very small switch.– Photography.

• The computer scientists got the idea of having layers of chemicals on top of each other from photography to “print” the wires of circuit directly on a non-conducting base and this led to invention of Integrated Circuits (ICs) which packs thousands of transistors on one chip.

Electrical Switches

• We adopt the convention that:

Electrical Switches

• We adopt the convention that:– A high voltage in a wire is represented by the symbol 1.– Little or no voltage in a wire represented by symbol 0.

Electrical Switches

• We adopt the convention that:– A high voltage in a wire is represented by the symbol 1.– Little or no voltage in a wire represented by symbol 0.

• With this convention a switch can be viewed as a logic operator.

Electrical Switches

• We adopt the convention that:– A high voltage in a wire is represented by the symbol 1.– Little or no voltage in a wire represented by symbol 0.

• With this convention a switch can be viewed as a logic operator.

• Think of a switch as box with three wires connected to it called in, out and control.

Electrical Switches

• We adopt the convention that:– A high voltage in a wire is represented by the symbol 1.– Little or no voltage in a wire represented by symbol 0.

• With this convention a switch can be viewed as a logic operator.

• Think of a switch as box with three wires connected to it called in, out and control.

• Our switches will come in two basic varieties:

Electrical Switches

• We adopt the convention that:– A high voltage in a wire is represented by the symbol 1.– Little or no voltage in a wire represented by symbol 0.

• With this convention a switch can be viewed as a logic operator.

• Think of a switch as box with three wires connected to it called in, out and control.

• Our switches will come in two basic varieties: – Normally open– Normally closed

An electronic switch in general

Normally Open Switch

• In a normally open switch, current can pass from in to out only when there is a signal at the control wire.

Normally Open Switch

• In a normally open switch, current can pass from in to out only when there is a signal at the control wire.

• That is when control is 1.

Normally Open Switch

• In a normally open switch, current can pass from in to out only when there is a signal at the control wire.

• That is when control is 1.• Otherwise the value of out is 0.

Normally Open Switch

Normally Closed Switch

• In a normally close switch, current can pass from in to out unless there is a signal at the control wire.

Normally Closed Switch

• In a normally close switch, current can pass from in to out unless there is a signal at the control wire.

• That is when control is 0.

Normally Closed Switch

• In a normally close switch, current can pass from in to out unless there is a signal at the control wire.

• That is when control is 0.• Otherwise the value of out is 0.

Normally Closed Switch

Logic

• From computer science perspective we are not concerned with electrical currents and voltages.

Logic

• From computer science perspective we are not concerned with electrical currents and voltages.

• We adopted the convention that:

Logic

• From computer science perspective we are not concerned with electrical currents and voltages.

• We adopted the convention that:– A high voltage in a wire is represented by the symbol 1.– Little or no voltage in a wire represented by symbol 0.

Logic

• From computer science perspective we are not concerned with electrical currents and voltages.

• We adopted the convention that:– A high voltage in a wire is represented by the symbol 1.– Little or no voltage in a wire represented by symbol 0.

• With this convention a switch can be viewed as a logic operator.

Logic

• From computer science perspective we are not concerned with electrical currents and voltages.

• We adopted the convention that:– A high voltage in a wire is represented by the symbol 1.– Little or no voltage in a wire represented by symbol 0.

• With this convention a switch can be viewed as a logic operator.

• We can regard a switch as an operator that produces an output state depending only on the input states.

Logic

• Example: S = John and Merry are happy. P = John is happy. Q = Merry is happy. S = P AND Q .

Logic

• Example: S = John and Merry are happy. P = John is happy. Q = Merry is happy. S = P AND Q . • AND is a logical operator.• We refer to P and Q as components of logical

operator AND.

Logic

• Suppose we decide to interpret – 0 as false.– 1 as true.

Logic

• Suppose we decide to interpret – 0 as false.– 1 as true.

• Then the action of normally open switch is the same, under this interpretation, as that of AND logical operator.

Logic

• Suppose we decide to interpret – 0 as false.– 1 as true.

• Then the action of normally open switch is the same, under this interpretation, as that of AND logical operator.

• Logical AND operator is evaluated to true if and only if both of its operands are true.

Logic

• A normally open switch acts as an AND operator.

• We refer to it as an AND gate. It has three components:– two input lines.– one output line.

AND Gate

AND Gate

OR gate

• OR is another logical operator. We also have a gate for OR and we call it OR gate.

OR gate

• OR is another logical operator. We also have a gate for OR and we call it OR gate.

• An OR gate like AND gate has three components:– Two input lines– One output line

OR gate

• OR is another logical operator. We also have a gate for OR and we call it OR gate.

• An OR gate like AND gate has three components:– Two input lines– One output line

• A Logical OR operator evaluates as true if at least one of the operands are true.

OR gate

• OR is another logical operator. We also have a gate for OR and we call it OR gate.

• An OR gate like AND gate has three components:– Two input lines– One output line

• A Logical OR operator evaluates as true if at least one of the operands are true.

• In the same line, an OR gate has output 1 if at least one of the input lines are 1.

OR Gate

NOT Gate• We have logical operator NOT which reverses

the truth value of its statement. Ex: R = Peter is happy. then, NOT R = Peter is not happy.

NOT Gate• We have logical operator NOT which reverses the truth value

of its statement. Ex: R = Peter is happy. then, NOT R = Peter is not happy.• As two other logical operator we have a corresponding NOT

gate which has two components:– One input line– One output line

NOT gate

Logical Expressions

• We can use the three basic logical operators, AND, NOT and OR to build any logical expression by combining them.

EX: We have two named statements: P = Button A has been pushed. R = Button B has been pushed. We want to write an expression Q which is true when either both of the buttons have been pushed or none of them.

Example

P R Q

1 1 1

1 0 0

0 1 0

0 0 1

True = 1False = 0P = 1 means button A has been pushed.

Finding the appropriate Expression

P R PR P’ R’ P’R’ PR + P’R’

1 1 1 0 0 0 1

1 0 0 0 1 0 0

0 1 0 1 0 0 0

0 0 0 1 1 1 1

PR stands for P AND RP’ stands for NOT PM + N stands for M OR NThus, we have Q = PR + P’R’

Building a Logical Circuit

• Now we want to build the a circuit for our computer using AND, OR and NOT gates which corresponds to logical expression Q = PR + P’R’

• That is given inputs P and R (in 0 and 1) produces Q (in 0 and 1)

Big Picture

Step 1

Step 2

Step 2

Step 3

Step 3

Multiplexer

• A multi-way switch• A two way multiplexer has two input lines,

one select line and one output line.• The select line determines the current from

which input line should pass to output line.

Multiplexer

2-way MUXa b select output

1 1 1 1

1 1 0 1

1 0 1 1

1 0 0 0

0 1 1 0

0 1 0 1

0 0 1 0

0 0 0 0

2-way MUX

• Select input a if the select is 1.• Select input b if the select is 0.

2-way MUX

Decoder

• Decoder is a circuit which has a single input and multiple outputs. It has one or more select lines.

• Select lines determine the current of input line should pass to which of the output lines.

Decoder

Two-way Decoder

a s0 x1 x0

1 1 1 0

1 0 0 1

0 1 0 0

0 0 0 0