Page 68 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

Using Tronsistors to Build Primitive Logic tion^ on^ w 4

Once again, an OR gate is formed by inverting the output of a NOR with
a NOT, which means that a 2-input OR requires six transistors (Figure 6-8).

a 00 0
Y
01 1 a
-
OR 101 b
1 1 1

Figure 6-8. CMOS implementation
of a 2-input OR gate

XNOR and XOR Gates

The concepts of NAND, AND, NOR, and OR are relatively easy to under-
stand because they map onto the way we think in everyday life. For example, a
textual equivalent of a NOR could be: “If it’s windy or if it’s raining then I’m not
going out. ’)
By comparison, the concepts of XOR and XNOR can be a little harder to
grasp because we don’t usually consider things in these terms. A textual equiva-
lent of an XOR could be: “Ifit is windy and it’s not ruining, or if it’s not windy
and it is raining, then I will go out.” Although this does make sense (in a strange
sort of way), we don’t often find ourselves making decisions in this manner.
For this reason, it is natural to assume that XNOR and XOR gates would
be a little more difficult to construct. However, these gates are full of surprises,
both in the way in which they work and the purposes for which they can be
-
used For example, a 2-input XNOR can be implemented using only four
transistors (Figure 6-9).8

XNOR 100
Tr.4
Figure 6-9. CMOS implementation b
of a 2-iinput XNOR gate

8 Unlike AND, NAND, OR, and NOR gates, there are no such beasts as XNOR or XOR Frimitives
with more than two inputs. However, equivalent functions with more than two inputs can be
formed by connecting a number of 2-input primitives together.