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ECE 331 – Digital System Design Multi-level Logic Circuits and NAND-NAND and NOR-NOR Circuits (Lecture #8) The slides included herein were taken from the materials accompanying Fundamentals of Logic Design, 6 th Edition, by Roth and Kinney, and were used with permission from Cengage Learning.

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Fall 2010ECE 331 - Digital System Design2 Multi-level Logic Circuits

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Fall 2010ECE 331 - Digital System Design3 Multi-level Logic Circuits Thus far we have focused on the realization of optimal logic circuits through the derivation of Minimum Sum-of-Products expressions Minimum Product-of-Sums expressions Both forms of Boolean expressions are realized as two-level logic circuits AND-OR circuit ↔ SOP OR-AND circuit ↔ POS There are a maximum of two logic gates between every input and the output(s).

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Fall 2010ECE 331 - Digital System Design4 Multi-level Logic Circuits A two-level logic circuit is usually efficient for Boolean expressions of a few variables. However, as the number of inputs increases, a two- level logic circuit may encounter in 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 Field Programmable Gate Array (FPGA) Complex Programmable Logic Device (CPLD)

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Fall 2010ECE 331 - Digital System Design5 Multi-level Logic Circuits May require fewer logic gates than the logically equivalent two-level logic circuit. Reduced (silicon) area Decreased cost May require less complex wiring between logic gates Fewer literals results in fewer interconnecting wires Has 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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Fall 2010ECE 331 - Digital System Design6 Find a circuit of AND and OR gates to realize Consider solutions with two levels of gates and three levels of gates. Try to minimize the number of gates and the total number of gate inputs. Assume that all variables and their complements are available as inputs. f (a, b, c, d) = Ʃ m(1, 5, 6, 10, 13, 14) Multi-level Logic Circuits Example:

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Fall 2010ECE 331 - Digital System Design7 First, simplify f by using a Karnaugh map. Multi-level Logic Circuits Example: Minimum SOPMinimum POS

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Fall 2010ECE 331 - Digital System Design8 This leads directly to a two-level AND-OR logic circuit for the minimum SOP expression. Multi-level Logic Circuit Example:

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Fall 2010ECE 331 - Digital System Design9 And leads directly to a two-level OR-AND logic circuit for the minimum POS expression. Multi-level Logic Circuit Example:

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Fall 2010ECE 331 - Digital System Design10 Factoring the minimum SOP expression yields: f = c'd.(a' + b) + cd'.(a + b) Multi-level Logic Circuit Example: Which leads to the three-level OR-AND-OR logic circuit:

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Fall 2010ECE 331 - Digital System Design11 Multi-level Logic Circuit Example: To get a three-level circuit with an AND gate output, we partially multiply out the POS expression using (X + Y)(X + Z) = X + Y Z: f = [c + d(a′ + b)][c′ + d′(a + b)] This would require four levels of gates to realize; however, if we multiply out d′(a + b) and d(a′ + b), we get f = (c + a′d + bd)(c′ + ad′ + bd′)

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Fall 2010ECE 331 - Digital System Design12 Multi-level Logic Circuit Example: Which leads to the three-level AND-OR-AND logic circuit:

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For this particular example, the best two-level solution had an AND gate at the output, and the best three-level solution had an OR gate at the output. In general, to be sure of obtaining a minimum solution, one must find both the circuit with the AND-gate output and the one with the OR-gate output. Multi-level Logic Circuits

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Fall 2010ECE 331 - Digital System Design14 NAND-NAND and NOR-NOR Circuits

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Fall 2010ECE 331 - Digital System Design15 Logic Gates AND and OR Gates 2-input gates realized with 6 CMOS transistors 3-input gates realized with 8 CMOS transistors NAND and NOR Gates 2-input gates realized with 4 CMOS transistors 3-input gates realized with 6 CMOS transistors More cost efficient to design logic circuits using NAND and NOR gates.

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Fall 2010ECE 331 - Digital System Design16 NAND and NOR Gates F = (ABC)′ = A′ + B′ + C′ F = (A + B + C)′ = A′B′C′

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Fall 2010ECE 331 - Digital System Design17 Any logic function can be realized using only NAND gates. Consequently, it is said to be a functionally complete set of gates. NAND Gate

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Fall 2010ECE 331 - Digital System Design18 Any logic function can be realized using only NOR gates. Consequently, it, too, is said to be a functionally complete set of gates. NOR Gate

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Fall 2010ECE 331 - Digital System Design19 A two-level circuit composed of AND and OR gates is easily converted to a circuit composed of NAND or NOR gates only. Use F = (F′)′ and then apply DeMorgan′s laws: (X 1 + X 2 + … + X n )′ = X 1 ′ X 2 ′…X n ′ (X 1 X 2 …X n )′ = X 1 ′ + X 2 ′ + … + X n ′ NAND-NAND and NOR-NOR Circuits

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Fall 2010ECE 331 - Digital System Design20 Convert the minimum sum-of-products (AND-OR) form to the equivalent NAND-NAND form. F = A + BC′ + B′CD = [(A + BC′ + B′CD)′ ]′ F = A + BC′ + B′CD = [A′ (BC′)′ (B′CD)′]′ NAND-NAND Circuits Example: Solution:

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Fall 2010ECE 331 - Digital System Design21 Convert the minimum product-of-sums (OR-AND) form to the equivalent NOR-NOR form. F = (A + B+ C)(A + B′ + C')(A +C' +D) = {[(A + B + C)(A + B′ + C′)(A + C′ + D)]′ }′ F = [(A + B + C)′ + (A + B′ + C′)′ + (A + C′ + D)′]′ NOR-NOR Circuits Example: Solution:

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Fall 2010ECE 331 - Digital System Design22 Basic Forms for Two-level Circuits

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Fall 2010ECE 331 - Digital System Design23 Basic Forms for Two-level Circuits

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Fall 2010ECE 331 - Digital System Design24 Design of NAND-NAND Circuits Find a minimum SOP expression for f. Draw the corresponding AND-OR circuit. Replace all gates with NAND gates, leaving the gate interconnection unchanged. If the output gate has any single literals as inputs, complement these literals.

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Fall 2010ECE 331 - Digital System Design25 Design a NAND-NAND Circuit Example:

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Fall 2010ECE 331 - Digital System Design26 Design of NOR-NOR Circuits Find a minimum POS expression for f. Draw the corresponding OR-AND circuit. Replace all gates with NOR gates, leaving the gate interconnection unchanged. If the output gate has any single literals as inputs, complement these literals.

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Fall 2010ECE 331 - Digital System Design27 Design a NOR-NOR Circuit Example:

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28 Design of Multi-level NAND-Gate Circuits Simplify the switching function to be realized. Design a multi-level circuit of AND and OR gates. The output gate must be an OR gate. The gates must alternate: AND, OR, AND, OR, … Number the levels starting with the output gate. The output gate is level 1. Replace all gates with NAND gates, leaving interconnections between gates unchanged. Leave inputs to gates at levels 2, 4, 6, … unchanged; invert literals that appear as inputs to gates at levels 1, 3, 5, …

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Fall 2010ECE 331 - Digital System Design29 Design of a Multi-level NAND-Gate Circuit

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30 Design of Multi-level NOR-Gate Circuits Simplify the switching function to be realized. Design a multi-level circuit of AND and OR gates. The output gate must be an AND gate. The gates must alternate: OR, AND, OR, AND, … Number the levels starting with the output gate. The output gate is level 1. Replace all gates with NOR gates, leaving interconnections between gates unchanged. Leave inputs to gates at levels 2, 4, 6, … unchanged; invert literals that appear as inputs to gates at levels 1, 3, 5, …

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Fall 2010ECE 331 - Digital System Design31 Design of a Multi-level NOR-Gate Circuit

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Fall 201032 Logic designers who design complex digital systems often find it convenient to use more than one representation for a given type of gate. Alternate Logic Gate Symbols inverter

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ECE 331 - Digital System Design33 Conversion to NAND-Gate Circuit AND-OR NAND-NAND

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Fall 2010ECE 331 - Digital System Design34 Conversion to NOR-Gate Circuit

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Fall 2010ECE 331 - Digital System Design35 Conversion to NAND-Gate Circuit

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Fall 2010ECE 331 - Digital System Design36 Multiple-output Circuits

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Fall 2010ECE 331 - Digital System Design37 Review the material provided in the textbook. (Sections 7.6 – 7.7) Multiple-output Logic Circuits

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Fall 2010ECE 331 - Digital System Design38 Questions?

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