Chapter 3 Notes – Part II
Review Questions
• What is the only input combination that will produce a HIGH at the output of a five-input AND gate?
• What logic level should be applied to the second input of a two-input AND gate if the logic signal at the first input is to be inhibited(prevented) from reaching the output?
• True or false: An AND gate output will always differ from an OR gate output for the same input conditions.

NOT operation
• Truth table, Symbol, Sample waveform

Summary of Boolean Operations
• OR
0+0=0
• AND
0•0=0
• NOT
1’=0
0+1=1 1+0=1 1+1=1
0•1=0 1•0=0 1•1=1
0’=1 (NOTE THE SYMBOL USED FOR NOT!)

3-6 Describing logic circuits algebraically
• Any logic circuit, no matter how complex, can be completely described using the three basic Boolean operations: OR,
AND, NOT.
• Example: logic circuit with its Boolean expression

(Often needed to establish precedence; sometimes used optionally for clarity)
• How to interpret A  B+C?
– Is it A  B ORed with C ?
– Is it A ANDed with B+C ?
• Order of precedence for Boolean algebra: AND before OR.
Parentheses make the expression clearer, but they are not needed for the case on the preceding slide.
• Note that parentheses are needed here :

Circuits Contains INVERTERs
• Whenever an INVERTER is present in a logic-circuit diagram, its output expression is simply equal to the input expression with a bar over it.

More Examples

Precedence
• First, perform all inversions of single terms
• Perform all operations with paretheses
• Perform an AND operation before an OR operation unless parentheses indicate otherwise
• If an expression has a bar over it, perform the operations inside the expression first and then invert the result

Determining output level from a diagram
Determine the output for the condition where all inputs are LOW.

3-8 Implementing Circuits From
Boolean Expressions
• When the operation of a circuit is defined by a Boolean expression, we can draw a logic-circuit diagram directly from that expression.

Example
• Draw the circuit diagram to implement the expression x

( A

B )( B

C )

Review Question
• Draw the circuit diagram that implements the expression x

A BC ( A

D )
Using gates having no more than three inputs.

3-9 NOR GATES AND NAND GATES
• NOR Symbol, Equivalent Circuit, Truth Table

Example

Example
• Determine the Boolean expression for a three-input NOR gate followed by an INVERTER

NAND Gate
• Symbol, Equivalent circuit, truth table

Example

Example
• Implement the logic circuit that has the expression x

AB


C

D
 using only NOR and NAND gates

Example
• Determine the output level in last example for A=B=C=1 and
D=0

Review Questions
• What is the only set of input conditions that will produce a HIGH output from a three-input NOR gate?
• Determine the output level in last example for A=B=1, C=D=0
• Change the NOR gate at last example to a NAND gate, and change the NAND to a NOR. What is the new expression for x?

3-10 Boolean Theorems
(single-variable)

Multivariable Theorems x+y = y+x
(x+y) + z = x + (y + z) x(y+z) = xy + xz x + xy = x pf: xy = yx
(xy)z = x(yz) commutativity associativity x + yz = (x+y) (x+z) distributivity x+xy = x1 + xy = x(1+y) = x1 = x

Examples
• Simplify the expression
• Simplify z


A

B


A

B

• Simplify x

ACD

A BCD y

A B D

A B D

Review Questions
• Simplify y

A C

AB C
• Simplify y

A B C D

A B C D
• Simplify y

A D

ABD

3-11 Demorgan’s Theorems
 x
 y

 x
 y
 x
 y

 x
 y

Example
• single variables inverted.

   

Implications of DeMorgan’s
Theorems(I)

Implications of DeMorgan’s
Theorems(II)

Example
• Determine the output expression for the below circuit and simplify it using DeMorgan’s Theorem

Review Questions
• Using DeMorgan’s Theorems to convert the expressions to one that has only single-variable inversions.
z


A

B


C y

R S T

Q
• Use only a NOR gate and an INVERTER to implement a circuit having output expression: z

A B C
• Use DeMorgan’s theorems to convert below expression to an expression containg only single-variable inversions.
y

A

B

C D

3-12 Universality of NAND and
NOR gates

Universality of NOR gate

Example

Example

3-13 Alternate Logic-Gate
Representations
• Standard and alternate symbols for various logic gates and inverter.

How to obtain the alternative symbol from standard ones
• Invert each input and output of the standard symbol, This is done by adding bubbles(small circles) on input and output lines that do not have bubbles and by removing bubbles that are already there.
• Change the operation symbol from AND to OR, or from OR to
AND.(In the special case of the INVERTER, the operation symbol is not changed)

Several points
• The equivalences can be extended to gates with any number of inputs.
• None of the standard symbols have bubbles on their inputs, and all the alternate symbols do.
• The standard and alternate symbols for each gate represent the same physical circuit; there is no difference in the circuits represented by the two symbols.
• NAND and NOR gates are inverting gates, and so both the standard and the alternate symbols for each will have a bubble on either the input or the output, AND and OR gates are noninverting gates, and so the alternate symbols for each will have bubbles on both inputs and output.

Logic-symbol interpretation
• Active high/low
– When an input or output line on a logic circuit symbol has no bubble on it, that line is said to be activehigh, otherwise it is active-low.

Interpretation of the two NAND gate symbols

Interpretation of the two OR gate symbols

Review Questions
• Write the interpretation of the operation performed by the below gate symbols
– Standard NOR gate symbol
– Alternate NOR gate symbol
– Alternate AND gate symbol
– Standard AND gate symbol