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Logic Gate Level Combinational Circuits, Part 1

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Circuits Circuit: collection of devices physically connected by wires to form a network Net can be: – combinational: input determines output – sequential: output not entirely dependent on input We will focus first on combinational networks Black box view of network Inputs: a, b, c Outputs: x, y

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Describing combinational networks 3 common methods for describing combinational networks, ordered from most to least abstract: – Truth tables – Boolean expressions – Logic diagrams

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Truth tables Most like black box view: – specifies what network does, not how – lists output for every possible combination of input values Next slide illustrates truth table for a 3-input, 2-output combinational network Note that the table has 8 entries – in general, n inputs translates to 2 n entries

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Truth table abcxy 00000 00110 01000 01111 10001 10100 11000 11100

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Boolean algebra A Boolean algebraic expression describes both what a network does and how it does it Boolean values: 1 = true 0 = false Binary operations in order of precedence: = AND (may be omitted; ab same as a b) + = OR Unary operation: ’ = NOT

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Fundamental properties of Boolean algebra Commutativea + b = b + aab =ba Associative(a + b) + c = a + (b + c)(ab)c = a(bc) Distributivea + (bc) = (a + b)(a + c)a(b + c) = ab + ac Identitya + 0 = a a 1 = a Complementa + a’ = 1 a a’ = 0 Note, in particular, how the distributive property of Boolean algebra differs from regular algebra – in regular algebra, multiplication (denoted with ) distibrutes over addition (+), but not vice versa, as the operations represented by these symbols (AND and OR) do in Boolean algebra

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Duality dual Boolean algebra has symmetry that doesn’t exist in regular algebra: every property (and thus every expression) has a dual To construct the dual of an expression: – exchange + and operations – exchange 1 and 0 Examples – the dual of c(a + b) is c + ab – the dual of x + 0 is x 1

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Simplifying expressions Duality is a useful property in the simplification of expressions Associative properties also permit simplifications; when a single operator is involved in a compound expression, parentheses can be removed Example: – (a + b) + c = a + b + c

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Idempotent property Weird and fancy way to say that any value ORed with itself yields the same value: x + x = x Proof: x + x = (x + x) 1 by the identity property (x + x) 1 = (x + x) (x + x’) by complement (x + x) (x + x’) = x + (x x’) by distribution x + (x x’) = x + 0 by complement x + 0 = x by identity Same sequence of steps can be used to prove the dual – so any value ANDed with itself will also yield the same value

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Notes on proofs A proof is a sequence of statements in which substitutions based on the fundamental properties of Boolean algebra are used to demonstrate the validity of a theorem Once a theorem is proven, we can immediately assert that its dual is true

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More Boolean algebra properties Absorption: – x + 1 = 1 – x 0 = 0 – x + xy = x – x(x + y) = x DeMorgan’s Laws: – (ab)’ = a’ + b’ – (a + b)’ = a’b’

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More Boolean algebra properties To prove that one expression is the complement of another, show that: – expr1 + expr2 = 1 and – expr1 expr2 = 0 DeMorgan’s laws: – illustrate proving complements – generalize to more than two variables – e.g. (abc)’ = a’ + b’ + c’ Complements of complements: – (x’)’ = x – 1’ = 0 – 0’ = 1

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Another useful operation Although any Boolean expression can be written as the combination of AND, OR and NOT operations, other operations are common The XOR (exclusive or) operation, denoted by the symbol has the following truth table for 2 variables (and generalizes, as the other operations do, to more than 2): ab a b 110 101 011 000 XOR takes precedence over OR, has lower precedence than AND

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Logic diagrams Logic diagrams are drawings representing the most detailed version of a circuit Each Boolean operation (AND, OR, NOT) is represented by a gate symbol with multiple inputs and a single output The lines in the drawing represent the wires in the circuit

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Three basic logic gates

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Logic diagrams It is possible to read a logic diagram as a kind of blueprint for a circuit, and to construct physical circuits from such blueprints It is actually far more common to use a slightly different set of gates in physical circuits, as these are easier to construct The next slide illustrates three of the most common gates

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Common logic gates NAND and NOR gates are logically equivalent to AND and OR gates followed by inverters. Since NAND is actually easier to build, ANDs are often constructed as inverted NAND gates.

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Relationship between various representations of combinational nets As the illustration below suggests, there is a one-to-one correspondence between a boolean expression and a logic diagram Each of these might correspond to a single line in a truth table

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Constructing logic diagram from Boolean expression For every draw a For every + draw a For every ’ draw a Show input(s) as line(s) coming into gate, output as line coming out Final output is output from final gate (not connected to input of another)

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Example: logic diagram for a + b’c

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More complicated expression The logic diagram above depicts ((ab + bc’)a)’ For a parenthesized expression, construct parenthesized portion first Note the connectors ( ) used to indicate a continuation of the same input (for a and b)

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Same expression, abbreviated Two inverters eliminated: first by including c’ as an input (instead of c) and second by making the third AND gate a NAND gate In this version, inputs a and b are shown multiple times – use same symbols, so we know they’re the same inputs

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One more example To translate logic diagram to Boolean expression: – label output of each gate (starting from left) with appropriate sub-expression – output of last gate is full expression

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