Programmable Logic Devices
are devices that can implement a wide variety of logic functions.
programming may be permanent or reprogrammable.
of common types of PROM's:
- (Read Only Memory) Is programmed by the manufacturer and can not be altered
by the user (you, the engineer).
-(Programmable ROM) can be programmed once. These are programmed by frying a
set of fuses in the device that permanently break connections between wires.
Thus, these devices can not be reprogrammed.
- (Erasable PROM) can be programmed and reprogrammed. To reprogram this device
you have to put it under ultraviolet light for an extended period of time.
(Electricallly Erasable PROM) This device can be erased electrically and is
therefore much easier and quicker to work with than a
other types of PLD's have similar technologies for programming them.
(I'll use this term generically to include all types of PROM's)
- Programmable Array Logic. This technology is obsolete so I will not discuss
Devices - Programmable Array Logic Devices. A very popular device for
implementing combinational logic, the type that we've been discussing.
Devices - Gate Array Logic. Similar to PAL Devices, but these have additional
- Programmable Gate Arrays. These are even more flexible than GAL's.
Field Programmable Gate Arrays. These devices are very elaborate and can be
reprogrammed while being in complete system.
Programmable Logic Devices
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.
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).
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.
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