1. Department of Preparatory Year, Umm Al Qura University CSCI 240 Digital Logic

2. Boolean Algebra to Logic Gates Logic circuits are built from components called logic gates. The logic gates correspond to Boolean operations +, *, ’. Binary operations have two inputs, unary has one OR + AND * NOT’

3. AND A B A*B Logic Gate: Series Circuit: ABABA*B000 010 100 111 Truth Table: A*B

4. A B A+B Logic Gate: Parallel Circuit: A B ABA+B 000 011 101 111 Truth Table: A+BOR

5. NOT A A’ or A Logic Gate: (also called an inverter) Single-throw Double-pole Switch: A aA 01 10 Truth Table: A’ or A

6. n -input Gates Because + and * are binary operations, they can be cascaded together to OR or AND multiple inputs. A B C A B C A+B+C A B A B C ABC C

7. n -bit Inputs For convenience, it is sometimes useful to think of the logic gates processing n -bits at a time. This really refers to n instances of the logic gate, not a single logic gate with n -inputs. 1101100101 0100110111 1101110111 10001111 00111100 00001100 110001 001110

8. Logic Circuits ≡ Boolean Expressions All logic circuits are equivalent to Boolean expressions and any boolean expression can be rendered as a logic circuit. AND-OR logic circuits are equivalent to sum-of-products form. Consider the following circuits: A C B abc aBc Ab y=abc+aBc+Ab y A B C Y y=aB+Bc Deduce the truth table for each

9. NAND and NOR Gates NAND and NOR gates can greatly simplify circuit diagrams. As we will see, can you use these gates wherever you could use AND, OR, and NOT. NAND NORAB ABABABAB001 011 101 110 AB ABABABAB001 010 100 110

10. XOR and XNOR Gates XOR is used to choose between two mutually exclusive inputs. Unlike OR, XOR is true only when one input or the other is true, not both. XOR XNOR AB ABABABAB 000 011 101 110 ABA B 001 010 100 111

11. Properties of NAND AND NOR NAND and NOR have special properties, but neither satisfies the distributive or associative laws. NANDNOR x  1=X x  0=X x  0=1 x  1=0 x  x=X x  x=X x  y=X+Y x  y=XY X  Y=x+y X  Y=xy not (x  y)=xy not (x  y)=x+y It should be clear by looking at these properties that NAND and NOR are duals.

12. NAND and NOR as Universal Logic Gates Any logic circuit can be built using only NAND gates, or only NOR gates. They are the only logic gate needed. Here are the NAND equivalents:

13. NAND and NOR as Universal Logic Gates (cont) Here are the NOR equivalents: NAND and NOR can be used to reduce the number of required gates in a circuit.

14. Example Problem A hall light is controlled by two light switches, one at each end. Find (a) a truth function, (b) a Boolean expression, and (c) a logic network that allows the light to be switched on or off by either switch. xy f(x,y) 000 011 101 110 (What kind of gate has this truth table? Let x and y be the switches: