ENSC 150

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ENSC 150
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• Logic Gates

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Agenda

• Physical Quantities
• Binary Signals
• Simple Logic Functions

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Physical Quantities

• Examples of physical quantities.

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A Simple Device
Sometimes information can flow in both directions
at a single port. In this case, the port is both
an input and an output.
Each port is associated with a single physical
quantity
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Example of a Device
Information enters in the form of an electrical
current
Outputs information in the form of light intensity
Consider a guitar string. If the length of the
string is changed then the frequency of
vibration (pitch) will also change.
If the length varies according to some distinct
pattern then the vibrations of the string will
also vary in some manner which is related to the
original input pattern. The information is the
pattern.
The information carried in the quantity length
is transformed to information carried in the
quantity pitch.
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Electrical Current
The conduction of objects which carry electrical
charge.
1) Pick any position along the path.
2) Now ask How much charge passes through this
point in one second.
The SI Units of electrical current are Coulombs
per Second
The Electrical Current is a physical quantity
which is defined at a single point along a
conducting path.
If the path has the special property that it
forms a loop whereby the current at every point
is equal then we may define a special object
called the Loop Current.
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Electrical Voltage
Units are Joules per Coulomb. energy per unit
of charge.
Voltage behaves like the physical quantity called
elevation.
Choose any two positions and then ask What is
the elevation of A relative to B?
The following questions are reasonably well
defined. What is the elevation of my position
above sea level? What is the elevations of the
café above sea level? What is the elevation of
the my position above the café?
Every path which returns to its starting point
has a total change in elevation of zero.
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Binary Signals
Given a port on some device, we must ask, Which
direction does the information flow? In, out or
both ways. What is the physical quantity
associated with this port?
We can break the range of possible values into 2
regions and label these regions 0,1 or
off,on or false, true or low, high etc.
In this case, the physical quantity is viewed as
carrying binary digital information as a posed to
analog information.
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TTL signals
The TTL standard states that the ports of a TTL
logic device carry information as voltages. The
ranges for acceptable 1s and 0s are different
for inputs and outputs.
For the remainder of this course we will study
systems which manipulate binary signals. We will
ignore the detailed physical nature of these
signals and simply assume that acceptable 1s
and 0s have been clearly defined.
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Logical Devices
Consider a device with no input ports and one
output port. what could such a device possibly
do?

• Constantly output a 1
• Constantly output a 0
• Randomly output 1s and 0s
• Alternate between 1 and 0 at a fixed
  frequency. etc

This device can be viewed as a generator of data.
(source)
Consider a device with one input port and no
output port. (sink) what could such a device
possible do? This is a useless device. bit
bucket
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More Logical Devices
Consider a device with one input port and one
output port.
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2-input Devices
How many possible ways could we fill in this
table?
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Possible 2-input devices
Lets first consider the simplest possible
devices.
These two devices are equivalent to the one box
and zero box from the earlier slide.
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Two ones
The first table could be called an ignore B box
since the output is always the same as input A.
The second table is also an ignore B box but
the output is always the COMPLEMENT of A.
The third table could be called an ignore A box
since the output is always the same as input B.
The second table is also an ignore A box but
the output is always the COMPLEMENT of B.
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More two ones
There are two more patterns that contain exactly
two 1s and two 0s.
The first table is a useful device which is given
the name Exclusive OR. It is a device which has
the ability to detect whether the values present
at the two inputs are either the same or
different.
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The last 8 patterns.
And now the patterns that contain exactly THREE
1s ( exactly ONE 0 )
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The OR and AND functions
If A has the truth OR B has the truth then the
truth comes out.
The truth only comes out when both A AND B have
the truth.
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Implementing Logic Gates
This configuration implements a 5-input OR
function.
What are the physical quantities associated with
each port?
Each switch is a single input port. The physical
quantity associated with this port could be the
pressure exerted by your finger.