Digital Circuits/NOR Logic

A single NOR gate

Like NAND gates, NOR gates are so-called "universal gates" that can be combined to form any other kind of logic gate. For example, the, Apollo Guidance Computer, was built exclusively from NOR gates, about 5,600 in total for the later versions. Today, contrary to popular belief, 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).

NOR

A NOR gate is trivially made:

Desired Gate	NOR Construction
Truth Table Input A Input B Output Q 0 0 1 0 1 0 1 0 0 1 1 0

NOT

This is made by joining the inputs of a NOR gate. As a NOR gate is equivalent to an OR gate leading to NOT gate, this automatically sees to the "OR" part of the NOR gate, eliminating it from consideration and leaving only the NOT part.

Desired Gate	NOR Construction
Truth Table Input A Output Q 0 1 1 0

OR

The OR gate is simply a NOR gate followed by a NOT gate.

Desired Gate	NOR Construction
Truth Table Input A Input B Output Q 0 0 0 0 1 1 1 0 1 1 1 1

AND

An AND gate gives a 1 output when both inputs are 1; a NOR gate gate gives a 1 output only when both inputs are 0. Therefore, an AND gate is made by inverting the inputs to a NOR gate.

Desired Gate	NOR Construction
Truth Table Input A Input B Output Q 0 0 0 0 1 0 1 0 0 1 1 1

NAND

A NAND gate is made using an AND gate in series with a NOT gate:

Desired Gate	NOR Construction
Truth Table Input A Input B Output Q 0 0 1 0 1 1 1 0 1 1 1 0

XOR

An XOR gate is made by connecting the output of 3 NOR gates (connected as an AND gate) and the output of a NOR gate to the respective inputs of a NOR gate. This expresses the logical fomula (A AND B) NOR (A NOR B). This construction entails a propagation delay three times that of a single NOR gate.

Desired Gate	NOR Construction
Truth Table Input A Input B Output Q 0 0 0 0 1 1 1 0 1 1 1 0

Alternatively, the 4-gate version of the XNOR gate can be used with an inverter. This construction has a propagation delay four times that of a single NOR gate, but uses the same number of gates.

Desired Gate	NOR Construction

XNOR

An XNOR gate can be constructed from four NOR gates implementing the expression "(A NOR N) NOR (B NOR N) where N = A NOR B".

Desired Gate	NOR Construction
Truth Table Input A Input B Output Q 0 0 1 0 1 0 1 0 0 1 1 1

Alternatively, the 5-gate version of the XOR gate can be used with an inverter. This construction has a propagation delay four times that of a single NOR gate, and uses more gates.

Desired Gate	NOR Construction

See also

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

Digital Circuits