Overview of Programmable Logic Devices

These are devices that are used in electronic circuits to make reconfiguration of the same possible. The logical gates that are normally used perform a fixed function however, the Programmable Logic Devices (PLD) does not perform any predefined task when they are manufactured and need to be programmed before they can be used.
Before the introduction of PLDs, ROM (Read Only Memories) was used. EPROM that was available could be programmed by using any standard hardware for that device and hence, did not require any other peripheral hardware. Another advantage of ROMs is that a particular binary code stored in it could be obtained on any of the n output lines that were available in it. Thus, they can be programmed in any way one wishes to. However, ROMs have some disadvantages when they are used as PLDs. They do not produce the outputs as quickly as other devices; they are not very dependable as far as asynchronous inputs are concerned, they require more amount of power etc.

The first PLD was the programmable logic array that was developed from ROAM (Read Only Associative Memory) by modification of one of its metal layers and had 17 inputs and 18 outputs.

Later another device was developed based on the PROM (Programmable Read Only Memory) technology. This device made use of the UV erasure technology that had been introduced recently. One of the features of this device was that it made implementation of multi-level logical equations possible.

Next to come was the Programmable Associative Logic Array that was capable of masking of certain inputs as and when required.

The programmable array logic is another type of PLD. These devices had a fixed array of OR gates and a programmable AND gate array. Thus, by making use of it, implementation of every possible function is not possible. On the other hand, the architecture of these devices is simple.

These devices were further worked upon and improved and thus, the Generic Array Logic (GAL) was formed. These devices were similar in functionality to the PALs but, could be erased and programmed again. They prove to be useful in debugging a particular program as they can be reprogrammed to suit ones needs if some errors are found in the logic that has already been programmed.

To implement logical equations on a large scale, complex PLDs are used. They are capable of taking a large number of inputs and can produce a large number of outputs as well. They can be programmed using the programmer used for PAL devices but, the process becomes tedious and hence, is usually avoided. Hence, it is usually programmed by soldering it to a circuit board and then, feeding the data into it by making use of a computer.

These devices prove to be very useful in the implementation of various circuits in digital signal processing as well. They can be used to apply various filters to the signal that has been converted into the digital form.