The first edition of this book started with the words: ‘A modern society could no longer function without the microprocessor.’
This is certainly still true but it is even truer if we include the microcontroller. While the microprocessor is at the heart of our computers, with a great deal of publicity, the microcontroller is quietly running the rest of our world. They share our homes, our vehicles and our workplace, and sing to us from our greetings cards. They are our constant, unseen companions and billions are being installed every year with little or no publicity. The purpose of this book is to give a worry-free introduction to microprocessors and microcontrollers. It starts at the beginning and does not assume any previous knowledge of microprocessors or microcontrollers and, in gentle steps, introduces the knowledge necessary to take those vital first steps into the world of the micro.
John Crisp
The NAND gate
The NAND gate
The word NAND is just a fancy contraction of NOT and AND. The NAND gate is just an AND gate followed by a NOT gate so all the outputs shown in the AND truth table are just inverted by the NOT gate.
On a diagram, this combination is indicated by putting a small circle on the end of the symbol. The symbol and truth table are shown in Figure 5.7.
Figure 5.7 The truth table for a NAND gate
It is better not to make a big effort to learn the NAND gate. Just remember that it’s the same as the AND gate except the output has been inverted by the NOT gate that has been added internally. The symbol has a line over the top to indicate the added NOT function. A two input NAND could be written as X=
or X=
or very occasionally X=
.
