ete 204 – digital electronics nand and nor circuits, multi-level logic circuits, and...

ETE 204 – Digital Electronics

NAND and NOR Circuits,Multi-level Logic Circuits,

andMultiple-output Logic Circuits

[Lecture: 8]Instructor: Sajib RoyLecturer, ETE, ULAB

Summer 2012 ETE 204 - Digital Electronics 2

NAND and NOR Circuits

3

CMOS Logic Gates

Inverter

2-input AND

2-input OR

2-input NAND2-input NOR

Summer 2012 ETE 204 - Digital Electronics

4

CMOS Logic Gates

# of inputs AND / OR NAND / NOR

2 6 4

3 8 6

4 10 8

5 12 10

Number of transistors per logic gate:

Thus, in terms of transistor count, it is “cheaper” to design logic circuits using NAND and NOR gates.


5

The NAND Gate

Any logic function can be realized using only NAND gates.

It is a functionally complete set of gates.


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The NOR Gate

Any logic function can be realized using only NOR gates.

It, too, is a functionally complete set of gates.

XX'

AB

(A+B)'

A+B

A

B

A'

B'

(A'+B')' = AB


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Alternate Logic Gate Symbols Digital circuit designers often find it convenient to use

more than one representation for a given logic gate.


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NAND and NOR Circuits

A two-level circuit composed of AND and OR gates is easily converted to a circuit composed of NAND or NOR gates only.

AND-OR → NAND-NAND OR-AND → NOR-NOR

To do so algebraically, First use F = (F')' Then apply DeMorgan's Theorem


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NAND Circuit: Example

Convert the following SOP expression from the AND-OR form to the NAND-NAND form:

F(A,B,C) = A.B' + A'.C' + B.C


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NOR Circuit: Example

Convert the following POS expression from the OR-AND form to the NOR-NOR form:

F(A,B,C) = (A'+B').(A'+C).(B+C')


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Design a NAND Circuit

• Find the minimum SOP expression for F.

• Draw the corresponding AND-OR circuit.

• Replace all gates with NAND gates, leaving the gate interconnection unchanged.

• Complement any literals connected directly to the output (OR) gate.


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NAND Circuit: Example

Design the NAND circuit for the following logic function:

F(A,B,C) = m(0, 3, 4, 5, 6, 7)


13

Design a NOR Circuit

• Find the minimum POS expression for F.

• Draw the corresponding OR-AND circuit.

• Replace all gates with NOR gates, leaving the gate interconnection unchanged.

• Complement any literals connected directly to the output (AND) gate.


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NOR Circuit: Example

Design the NAND circuit for the following logic function:

F(A,B,C) = m(2, 4)


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Multi-level Logic Circuits


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Multi-level Logic Circuits Thus far we have focused on the realization of optimal

logic circuits through the derivation of Minimum Sum of Products (SOP) expressions Minimum Product of Sums (POS) expressions

Both forms of Boolean expressions are realized as two-level logic circuits

SOP ↔ AND-OR (NAND-NAND) circuit POS ↔ OR-AND (NOR-NOR) circuit There are a maximum of two logic gates between every

input and the output(s).


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Multi-level Logic Circuits A two-level logic circuit is usually efficient for Boolean

expressions of a few variables (i.e. inputs). However, as the number of inputs increases, a two-

level logic circuit may encounter fan-in problems. Fan-in refers to the number of inputs to a logic gate

Whether fan-in is an issue is dependent upon the technology used to implement the logic circuit.

Standard TTL and CMOS chips Complex Programmable Logic Device (CPLD) Field Programmable Gate Array (FPGA)


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Multi-level Logic Circuits A multi-level logic circuit may require fewer logic gates

than the logically equivalent two-level logic circuit. Reduced (silicon) area Decreased cost

It may require less complex wiring between logic gates Fewer literals results in fewer interconnecting wires

It will have a greater propagation delay than the logically equivalent two-level logic circuit.

Each additional level adds to the propagation delay Decreased speed


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Multi-level Logic Circuits: Example

Design a logic circuit to realize the following logic function:

F(A,B,C) = m(1, 2, 3, 4, 6)

Given the following criteria:

1. Use AND and OR gates only2. Two- or Three-level circuit only3. Minimize the number of gates4. Minimize the number of gate inputs


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Multi-level Logic Circuits: Example

Which design would be possible if the following additional criteria was imposed:

5. Two-input logic gates only


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Multi-level Logic Circuitsusing

NAND and NOR gates


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Design a Multi-level NAND Circuit

• Derive a minimum expression for the logic function.

• Design a multi-level circuit using AND and OR gates. Output gate must be an OR gate.

Gates must alternate: AND, OR, AND, OR, …

• Number the levels starting with the output gate.

• Replace all gates with NAND gates, leaving the interconnection between gates unchanged.

• Leave inputs to gates at the even levels unchanged; complement inputs to gates at the odd levels.


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Design a Multi-level NAND Circuit


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Design a Multi-level NOR Circuit

• Derive a minimum expression for the logic function.

• Design a multi-level circuit using AND and OR gates. Output gate must be an AND gate.

Gates must alternate: OR, AND, OR, AND, …

• Number the levels starting with the output gate.

• Replace all gates with NOR gates, leaving the interconnection between gates unchanged.

• Leave inputs to gates at the even levels unchanged; complement inputs to gates at the odd levels.


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Design a Multi-level NOR Circuit


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Multiple-output Logic Circuits


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Thus far, we have focused on designing logic circuits to realize a single logic function.

A logic circuit with a single output. However, many logic circuits have multiple

outputs. Corresponding to multiple logic functions of

the same input variables.


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An optimal logic circuit may not be realized by simply minimizing each of the logic functions independently.

Rather, it may be necessary to consider which terms (i.e. logic gates), if any, are common to the logic functions to be realized.

Share those logic gates that are in common.


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Multiple-output Circuits: Example

Design an optimal logic circuit to realize the following logic functions:

F(A,B,C) = m(0, 4, 5)G(A,B,C) = m(0, 2, 6)

Cost = # of logic gates + # of gate inputs


30

Questions?


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