9
Programmable Logic Devices
Logic devices constitute one of the three important classes of devices used to build digital electronics
systems, memory devices and microprocessors being the other two. Memory devices such as ROM
and RAM are used to store information such as the software instructions of a program or the contents
of a database, and microprocessors execute software instructions to perform a variety of functions,
from running a word-processing program to carrying out far more complex tasks. Logic devices
implement almost every other function that the system must perform, including device-to-device
interfacing, data timing, control and display operations and so on. So far, we have discussed those
logic devices that perform fixed logic functions decided upon at the manufacturing stage. Logic
gates, multiplexers, demultiplexers, arithmetic circuits, etc., are some examples. Sequential logic
devices such as flip-flops, counters, registers, etc., to be discussed in the following chapters, also
belong to this category of logic devices. In the present chapter, we will discuss a new category
of logic devices called programmable logic devices (PLDs). The function to be performed by a
programmable logic device is undefined at the time of its manufacture. These devices are programmed
by the user to perform a range of functions depending upon the logic capacity and other features
offered by the device. We will begin with a comparison of fixed and programmable logic, and
then follow this up with a detailed description of different types of PLDs in terms of operational
fundamentals, salient features, architecture and typical applications. A brief introduction to the
devices offered by some of the major manufacturers of PLDs and PLD programming languages is
given towards the end of the chapter.
9.1 Fixed Logic Versus Programmable Logic
As outlined in the introduction, there are two broad categories of logic devices, namely fixed logic
devices and programmable logic devices. Whereas a fixed logic device such as a logic gate or
a multiplexer or a flip-flop performs a given logic function that is known at the time of device
manufacture, a programmable logic device can be configured by the user to perform a large variety of
Digital Electronics: Principles, Devices and Applications Anil K. Maini
© 2007 John Wiley & Sons, Ltd. ISBN: 978-0-470-03214-5
Programmable Logic Devices
Logic devices constitute one of the three important classes of devices used to build digital electronics
systems, memory devices and microprocessors being the other two. Memory devices such as ROM
and RAM are used to store information such as the software instructions of a program or the contents
of a database, and microprocessors execute software instructions to perform a variety of functions,
from running a word-processing program to carrying out far more complex tasks. Logic devices
implement almost every other function that the system must perform, including device-to-device
interfacing, data timing, control and display operations and so on. So far, we have discussed those
logic devices that perform fixed logic functions decided upon at the manufacturing stage. Logic
gates, multiplexers, demultiplexers, arithmetic circuits, etc., are some examples. Sequential logic
devices such as flip-flops, counters, registers, etc., to be discussed in the following chapters, also
belong to this category of logic devices. In the present chapter, we will discuss a new category
of logic devices called programmable logic devices (PLDs). The function to be performed by a
programmable logic device is undefined at the time of its manufacture. These devices are programmed
by the user to perform a range of functions depending upon the logic capacity and other features
offered by the device. We will begin with a comparison of fixed and programmable logic, and
then follow this up with a detailed description of different types of PLDs in terms of operational
fundamentals, salient features, architecture and typical applications. A brief introduction to the
devices offered by some of the major manufacturers of PLDs and PLD programming languages is
given towards the end of the chapter.
9.1 Fixed Logic Versus Programmable Logic
As outlined in the introduction, there are two broad categories of logic devices, namely fixed logic
devices and programmable logic devices. Whereas a fixed logic device such as a logic gate or
a multiplexer or a flip-flop performs a given logic function that is known at the time of device
manufacture, a programmable logic device can be configured by the user to perform a large variety of
Digital Electronics: Principles, Devices and Applications Anil K. Maini
© 2007 John Wiley & Sons, Ltd. ISBN: 978-0-470-03214-5