Page 29 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Digital Circuits and Logic Design

Notes 2.1.4 NAND Gate
The ”N“ letter on NAND stands for NOT, meaning that NAND logic gate is an AND gate with
an inverter attached. So, its output is the opposite from AND. Its symbol is the same of AND but
with a ”o“ on its output, meaning that the output is inverted. You can build yourself a NAND
gate by connecting an AND gate to an inverter.
Figure 2.9: NAND Logic Gate

Table 2.4: Truth table

A B Y
0 0 1
0 1 1
1 0 1
1 1 0

Another way to understand NAND logic gate: its output will only be at”0“ when all its inputs
are also at ”1“. Otherwise its output will be ”1“.
If you need more inputs, just draw them on the symbol shown in Figure 2.9. However, if you
want to create more inputs using gates with fewer inputs, you cannot connect them using the
same idea shown in Figure 2.4. You need to use AND gates for the “extra” inputs (you can link
them together to expand the number of inputs like shown in Figure 2.4) and a NAND gate for
the ”final“ gate, see Figure 2.10.
Figure 2.10: How to Expand the Number of Inputs of a NAND Gate

You can also easily transform NAND and NOR gates into inverters by shorting their inputs, like
shown in Figure 2.11. This is a very common trick. For example, you need an inverter in your
circuit and have some NAND gates available at a given integrated circuit. Instead of adding
another integrated circuit to your project just to have one inverter (what would increase the final
circuit size and also the cost), you may want to use one of the available NAND gates.
Figure 2.11: Transforming a NAND Gate into an Inverter

The most famous NAND gate integrated circuit is 7400 and you can its pinout in Figure 2.12. Of
course there are several other integrated circuits that provide NAND gates with more inputs.
For example, 7411 provides three three-input NAND gates and 7430 provides one eight-input
NAND gate.

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