ECE 331 – Digital System Design NAND and NOR Circuits, Multi-level Logic Circuits, and Multiple-output Logic Circuits (Lecture #9) 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.

Spring 2011ECE Digital System Design2 NAND and NOR Circuits

Spring 2011ECE Digital System Design3 CMOS Logic Gates Inverter 2-input AND 2-input OR 2-input NAND2-input NOR

Spring 2011ECE Digital System Design4 CMOS Logic Gates # of inputsAND / ORNAND / NOR Number of transistors per logic gate: Thus, in terms of transistor count, it is “cheaper” to design logic circuits using NAND and NOR gates.

Spring 2011ECE Digital System Design5 The NAND Gate Any logic function can be realized using only NAND gates. It is a functionally complete set of gates.

Spring 2011ECE Digital System Design6 The NOR Gate Any logic function can be realized using only NOR gates. It, too, is a functionally complete set of gates. X X' A B (A+B)' A+B A B A' B' (A'+B')' = AB

Spring 2011ECE Digital System Design7 Alternate Logic Gate Symbols Digital circuit designers often find it convenient to use more than one representation for a given logic gate.

Spring 2011ECE Digital System Design8 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

Spring 2011ECE Digital System Design9 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

Spring 2011ECE Digital System Design10 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')

Spring 2011ECE Digital System Design11 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.

Spring 2011ECE Digital System Design12 NAND Circuit: Example Design the NAND circuit for the following logic function: F(A,B,C) =  m(0, 3, 4, 5, 6, 7)

Spring 2011ECE Digital System Design13 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.

Spring 2011ECE Digital System Design14 NOR Circuit: Example Design the NAND circuit for the following logic function: F(A,B,C) =  m(2, 4)

Spring 2011ECE Digital System Design15 Multi-level Logic Circuits

Spring 2011ECE Digital System Design16 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).

Spring 2011ECE Digital System Design17 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)

Spring 2011ECE Digital System Design18 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

Spring 2011ECE Digital System Design19 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 only 2. Two- or Three-level circuit only 3. Minimize the number of gates 4. Minimize the number of gate inputs

Spring 2011ECE Digital System Design20 Multi-level Logic Circuits: Example Which design would be possible if the following additional criteria was imposed: 5. Two-input logic gates only

Spring 2011ECE Digital System Design21 Multi-level Logic Circuits using NAND and NOR gates

Spring 2011ECE Digital System Design22 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.

Spring 2011ECE Digital System Design23 Design a Multi-level NAND Circuit

Spring 2011ECE Digital System Design24 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.

Spring 2011ECE Digital System Design25 Design a Multi-level NOR Circuit

Spring 2011ECE Digital System Design26 Multiple-output Logic Circuits

Spring 2011ECE Digital System Design27 Multiple-output Logic Circuits 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.

Spring 2011ECE Digital System Design28 Multiple-output Logic Circuits 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.

Spring 2011ECE Digital System Design29 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

Spring 2011ECE Digital System Design30 Questions?