Universal Logic Gates

Introduction                             

A universal logic gate is a logic gate that can be used to construct all other logic gates.  There are many articles about how NAND and NOR are universal gates, but many of these articles omit other gates that are also universal gates. This article covers two input logic gates, demonstrates that the NAND gate is a universal gate, and demonstrates how other gates are universal gates that can be used to construct any logic gate.

Two Input Logic Gates

For 2 logical inputs, there are 16 possible logic gates. Table 1 defines the 16 logic gates, shows logical symbol, behavior in symbolic logic, provides name, notes.

People are most familiar with the ones that are named: NAND, NOR, AND, OR, XOR. Table 1 includes these, as well as AND and OR gates with either input negated, logic 0 and logic 1(which aren't really logic gates), and some gates that aren't really binary.

	Output when AB is
Gate #	00	01	10	11	Symbolic Logical Description	Symbol	Name/Description	Note
0	0	0	0	0	0		Always zero	Not really a gate
1	0	0	0	1	A & B		AND
2	0	0	1	0	A & ~B		A and Not B	Universal
3	0	0	1	1	A		A	Not Binary
4	0	1	0	0	~A & B		B and Not A	Universal
5	0	1	0	1	B		B	Not Binary
6	0	1	1	0	(A & ~B) | (~A & B)		XOR	Also Not Equals Function
7	0	1	1	1	A | B		OR
8	1	0	0	0	~(A | B)		NOR	Universal
9	1	0	0	1	(A & B) | (~A & ~B)		XNOR	Also Equals Function
10	1	0	1	0	~B		Not B	Not Binary
11	1	0	1	1	A | ~B		A or Not B	Universal
12	1	1	0	0	~A		Not A	Not Binary
13	1	1	0	1	~A | B		B or Not A	Unversal
14	1	1	1	0	~(A & B)		NAND	Universal
15	1	1	1	1	1		Always 1	Not really a gate

Table 1: 2 Input Logic Gates

NAND Gate is a Universal Gate

NAND gates can be connected to form any other logic gates. Figures 1,2,3 show how NAND gates can be connected to form INVERTER, AND, and OR gates. These gates can be combined to form the other logic gates according to the symbolic logic definitions in table 1.

Figure 1: Connecting a NAND gate to make an INVERTER gate

Figure 2: Connecting NAND gates to make an AND gate

Figure 3: Connecting NAND gates to make an OR gate

Other Universal Gates

Gates 2,4,11, and 13 are universal gates. To prove this, we demonstrate building NAND gates with these gates. Gates 7 and 8 are the same gate but with different inputs inverted. Gates 9 and 10 are the same gate but with different inputs inverted. Proving that gates 2 and 11 are universal is enough to prove that gate 4 and 13 are universal as well.

Gate 11: A | ~B

Figures 4 and 5 show show gate 11 can be combined to create INVERTER and AND gates. INVERTER and AND gates can be connected in series to construct a NAND gate. Because gate 11 can be used to construct a NAND gate, and a NAND gate is a universal gate, gate 11 is a universal gate. Since gates 11 and 13 are mirrors of each other, gate 13 is also a universal gate.

Figure 4: Connecting gate 11(A |~B) to create an INVERTER gate.

Figure 5: Connecting gate 11(A |~B) to create an AND gate. There may be a way to do this with fewer gates. The figure here is the one the author could derive the quickest.

Gate 2: A & ~B

Figures 6 and 7 show show gate 2 can be combined to create INVERTER and AND gates. INVERTER and AND gates can be connected in series to construct a NAND gate. Because gate 2 can be used to construct a NAND gate, and a NAND gate is a universal gate, gate 2 is a universal gate. Since gates 2 and 4 are mirrors of each other, gate 4 is also a universal gate.

Figure 6: Connecting gate 2(A &~B) to create an INVERTER gate.Figure 7: Connecting gate 2(A &~B) to create an AND gate.

Conclusion

This article has discussed logic gates, universal gates, demonstrated that the NAND gate is a universal gate, and proven that four additional universal logic gates exist. This is an interesting academic exercise. In practice, the author would recommend sticking to using NAND gates as universal gates because they are so cheap and small to construct, or if doing board level design using chips with the actual gates that are wanted and not using universal gates.