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Chapter: Digital Electronics : Memory Devices

Programmable Logic Devices(PLD)

PLD's are devices that can implement a wide variety of logic functions.

Programmable Logic Devices


·              PLD's are devices that can implement a wide variety of logic functions.

·              The programming may be permanent or reprogrammable.


·              Examples of common types of PROM's:


o         ROM - (Read Only Memory) Is programmed by the manufacturer and can not be altered by the user (you, the engineer).


o         PROM -(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.


o         EPROM - (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.


o          EEPROM - (Electricallly Erasable PROM) This device can be erased electrically and is therefore much easier and quicker to work with than a



·              The other types of PLD's have similar technologies for programming them.

·              Common PLD's include:

o          PROM's (I'll use this term generically to include all types of PROM's)


o         PLA's - Programmable Array Logic. This technology is obsolete so I will not discuss it.


o         PAL Devices - Programmable Array Logic Devices. A very popular device for implementing combinational logic, the type that we've been discussing.


o         GAL Devices - Gate Array Logic. Similar to PAL Devices, but these have additional flexibility.

o         PGA - Programmable Gate Arrays. These are even more flexible than GAL's.


o          FPGA's - Field Programmable Gate Arrays. These devices are very elaborate and can be reprogrammed while being in complete system.


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.


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