< Digital Circuits
A single NAND gate

Like NOR gates, NAND gates are so-called "universal gates" that can be combined to form any other kind of logic gate. Today, integrated circuits are not constructed exclusively from a single type of gate. Instead, EDA tools are used to convert the description of a logical circuit to a netlist of complex gates (standard cells) or transistors (full custom approach)

Standard gates constructions with a NOR gates combination

NAND

This is a NAND gate: trivially, just a NAND gate is used to realise it:

Desired GateNAND Construction
AB Q
001
011
101
110

NOT

A NOT gate is made by joining the inputs of a NAND gate. Since a NAND gate is equivalent to an AND gate followed by a NOT gate, joining the inputs of a NAND gate leaves only the NOT part.

Desired GateNAND Construction
AQ
01
10

AND

An AND gate is made by following a NAND gate by a NOT gate as shown below. This gives a NOT NAND, i.e. AND.

Desired GateNAND Construction
AB Q
000
010
100
111

OR

If the truth table for a NAND gate is examined, it can be seen that if any of the inputs are 0, then the output will be 1. However to be an OR gate, if any input is 1, the output must also be 1. Therefore, if the inputs are inverted, any high input will trigger a high output.

Desired GateNAND Construction
AB Q
000
011
101
111

NOR

A NOR gate is simply an OR gate with an inverted output:

Desired GateNAND Construction
AB Q
001
010
100
110

XOR

An XOR gate can be constructed from four NAND gates implementing the expression (A NAND N) NAND (B NAND N) where N = A NAND B. This construction entails a propagation delay three times that of a single NAND gate and uses four gates.

Desired GateNAND Construction
AB Q
000
011
101
110

Alternatively, an XOR gate is made by considering the disjunctive normal form , noting from de Morgan's Law that a NAND gate is an inverted-input OR gate. This construction uses five gates instead of four.

Desired GateNAND Construction

XNOR

An XNOR gate is made by considering the disjunctive normal form , noting from de Morgan's Law that a NAND gate is an inverted-input OR gate. This construction entails a propagation delay three times that of a single NAND gate and uses five gates.

Desired GateNAND Construction
AB Q
001
010
100
111

Alternatively, the 4-gate version of the XOR gate can be used with an inverter. This construction has a propagation delay four times (instead of three times) that of a single NAND gate.

Desired GateNAND Construction

See also

  • NOR logic. Like NAND gates, NOR gates are also universal gates.
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