12-12-2012, 05:57 PM
LOGIC GATES
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Most Difficult Reading Topics
• Logic gates and figuring out how to read
them
• Logical Circuit Equivalence
• NAND NOR and XOR truth tables
• Using the rules to create and read the
logic gates using 0's and 1's
• Transistor implementation
• Difference between positive logic and
negative logic
Logic
• Formal logic is a branch of
mathematics that deals
with true and false values
instead of numbers.
• In the mid-19th century, George Bool
developed many Logic ideas.
• Boolean logic deals with equations where
the operators are “AND” or “OR” instead of
“add” and “multiply”.
Electric Logic
• Logical values can easily be expressed by
an electrical circuit.
• “True” or “1” can be defined as voltage on a
wire while “False” or “0” can be defined as
no voltage. We will use positive logic.
• Analog values can be anything while digital
only has discrete values, 0 or 1
• Electrical devices called “gates” can
implement the logical
Inputs and Outputs
• The inputs can be considered as logical
(true or false) values.
• You can also think of the inputs as voltage
(true) or ground (false).
• The output of the gate will also be voltage
(true) or ground (false).
Three Input AND Gate
• Here is an example of a three input
AND gate.
• Notice that the truth table for the
three input gate is similar to the truth
table for the two input gate.
• It works on the same principle, this
time all three inputs need to be true
(1) to get a true output.
Four Input AND Gate
• Here is an example of a four input
AND gate.
• It also works on the same principle,
all four inputs need to be true (1) to
get a true output.
• The same principles apply to 5, 6,..., n
input gates.