# Introduction to Combinational Logic Functions

#### Chapter 9 - Combinational Logic Functions

The term “combinational” comes to us from mathematics. In mathematics a

combination is an unordered set, which is a formal way to say that nobody cares

which order the items came in. Most games work this way, if you rolled dice one

at a time and get a 2 followed by a 3 it is the same as if you had rolled a 3

followed by a 2. With combinational logic, the circuit produces the same output

regardless of the order the inputs are changed.

There are circuits which depend on the when the inputs change, these

circuits are called sequential logic. Even though you will not find the term

“sequential logic” in the chapter titles, the next several chapters will discuss

sequential logic.

Practical circuits will have a mix of combinational and sequential logic,

with sequential logic making sure everything happens in order and combinational

logic performing functions like arithmetic, logic, or conversion.

You have already used combinational circuits. Each logic gate discussed

previously is a combinational logic function. Let’s follow how two NAND gate

works if we provide them inputs in different orders.

We begin with both inputs being 0.

We then set one input high.

We then set the other input high.

So NAND gates do not care about the order of the inputs, and you will find

the same true of all the other gates covered up to this point (AND, XOR, OR,

NOR, XNOR, and NOT).

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