ch 1 Intro to digital logic student version

# ch 1 Intro to digital logic student version - EC262 Topic 2...

This preview shows pages 1–4. Sign up to view the full content.

1 EC262 Topic 2: Introduction to Combinational Systems Much of a computer’s hardware is comprised of digital logic circuits. Digital logic circuits are built from just a handful of primitive elements, called logic gates, combined in various ways. In a digital logic circuit, only two values may be present. The values may be 5 and + 5 volts. Or the values may be 0.5 and 3.5 volts. Or the values may be… you get the picture. To allow consideration of all of these possibilities, we will say that digital logic circuits allow the presence of two logical values: 0 and 1. So, signals in a digital logic circuit take on the values of 0 or 1. Logic gates are devices which compute functions of these binary signals. The Three Basic Logic Gates AND Gate . Consider the circuit below which has 2 switches in series: Switch S1 Switch S2 Light When will the light turn on? We can represent the light's operation using a table: S1 S2 Light open open off open shut off shut open off shut shut on Change the words open and off to 0 and the words shut and on to 1 and the table becomes S1 S2 Light This is the truth table for the AND operation. 0 0 0 0 1 0 1 0 0 1 1 1 The simplest AND gate gas two inputs and one output. If both inputs are 1 then the output is 1 . Otherwise, the output is 0 . The symbol for the AND gate is: We represent the AND operation by using either the multiplication symbol (i.e., “ “) or by writing the inputs together. Thus, for the AND gate above, we would write the output c as c a b = or as c = ab . This would be pronounced: “ C = a and b .”

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document