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
AND Gate
• 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
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.
P
R
Q
1
1
1
1
0
0
0
1
0
0
0
1
Example
True = 1
False = 0
P = 1 means button A has been
pushed.
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
Finding the appropriate Expression
PR stands for P AND R
P’ stands for NOT P
M + N stands for M OR N
Thus,
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 MUX
a
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