Programmable Logic Devices (plds)

The purpose of a PLD device is to permit elaborate digital logic designs to be implemented by the user in a single device

. It can be erased electrically and reprogrammed with a new design, making them very well suited for academic and prototyping. There are different types of Programmable Logic Devices available in the market they are SPLDs (Simple Programmable Logic Devices), ROM (Read-Only Memory), PLA (Programmable Logic Array), PAL (Programmable Array Logic), GAL (Generic Array Logic), CPLD (Complex Programmable Logic Device) and FPGA (Field-Programmable Gate Array). Newer variants are also available, but these are the most basic PLDs out there. The first three varieties are quite similar to each other; they all have an input connection matrix, which connects the inputs of the device to an array of AND-gates. They also have an output connection matrix, which connect the outputs of the AND-gates to the inputs of OR-gates which drive the outputs of the device. The gate array type of PLD is however very different in architecture and working as compared to the former ones stated here. Here the transistor or the logic elements are placed at predefined locations on the chip. In the gate setup logic gates are used very extensively as compared to other types. The different types of PLDs vary with the extent up to which the programmer could change to matrices.

A Programming Logic Device consists of logic built-in blocks (LBB) connected with a programmable interconnection array (PIA). Each LBB has two configurable logic cells sharing a group of control product terms, which serve as global and local control signals. They consists of a networking connection of OR, AND gates. Logic buffers are also present along with inverters. These smaller modules inside a PLD are often connected in flip flop configuration in order to enable storage of data. Hardware designs for very simple PLDs are generally written in languages like ABEL or PALASM. They are similar to hardware assembly of a system.

PLD can be logically connected and be used to create logic circuits either by masking or electrically programmable connection or by simple wire connection. The advantages which they posses over any other type of configuration is that first and foremost they are programmable thus enabling the user to make the same device behave different differently as per requirement by inputting the desired program into the device. Also they are power effective and do not contribute too much losses. With programmable logic devices, designers use inexpensive software tools to quickly develop, simulate, and test their designs. Then, a design can be quickly programmed into a device, and immediately tested in a live circuit. Programming Logic Devices also offer different numbers of fan in and fan out options .i.e. the number of inputs and outputs of the device.

Nowadays there has been a huge rise in development towards the hybrid chips. They combine a core processor with a portion of programmable logic circuitry. This has several implications; they are a combination of both fixed as well as programmable logics thus combining the best of both.

by: Varun

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