Page 197 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

Programmable ICs

Programmable integrated circuits are standard devices constructed in such
a way that a designer can configure them to perform a specific function. These
devices, which were introduced in the mid-1970s) are generically known as
pogrummuble logic devices (PLDs),~,~ and can range in complexity from a few
hundred to tens of thousands of equivalent gates. Traditional PLDs are based
on logic gates formed from diodes (Figure 16-1).

2-Input AND VDD (Logic 1)
2-1 n put OR

r b x = a & b r b ~ ~ RY y = a l b

~55 (Logic 0)
Figure 16-1. Diode implementations of AND and OR gates

In the case of the 2-input AND, the pull-up resistor % attempts to hold
the output y at VDD (logic I). When both of the inputs are at logic I, there is no
difference in potential across the diodes and y remains at logic 1. However, if
input a is connected to v,, (logic 0)) diode D, will conduct and y will be pulled
down to logic 0; similarly for input b and diode D,. Additional inputs can be
formed by adding more diodes.
In the case of the 2-input OR, the pull-down resistor % attempts to hold
the output y at V,, (logic 0). When both of the inputs are at logic 0, there is
no difference in potential across the diodes and y remains at logic 0. However,

1 In conversation, PLD is pronounced by spelling it out as “P-L-D.”
2 Originally all PLDs contained a modest number of equivalent logic gates and were fairly simple.
As more complex PLDs (CPLDs) arrived on the scene, however, it became common to refer to
their simpler cousins as simple PLDs (SPLDs). (Note that CPLDs are introduced in more detail
later in this chapter.)