Programmable Logic Devices (PLD)

In the world of digital electronic systems, there are three basic kinds of devices: memory, microprocessors, and logic. Memory devices store random information such as the contents of a spreadsheet or database. Microprocessors execute software instructions to perform a wide variety of tasks such as running a word processing program or video game. Logic devices provide specific functions, including device-to-device interfacing, data communication, signal processing, data display, timing and control operations, and almost every other function a system must perform.

Programmable Logic – a logic element whose function is not restricted to a particular function.  It may be programmed at different points of the life cycle.  At the earliest, it is programmed by the semiconductor vendor (standard cell, gate array), by the designer prior to assembly, or by the user, in circuit.

Programmable Logic Devices (PLDs) are ICs with a large number of gates and flip flops that can be configured with basic software to perform a specific logic function or perform the logic for a complex circuit. Unlike a logic gate, which has a fixed function, a PLD has an undefined function at the time of manufacture. Before the PLD can be used in a circuit it must be programmed, that is, reconfigured.

Advantages of PLDs:-

programmable logic devices offer a number of important advantages over fixed logic devices, including:

• Design Flexibility: PLDs offer customers much more flexibility during the design cycle because design iterations are simply a matter of changing the programming file, and the results of design changes can be seen immediately in working parts.
• Improved Reliability :Lower power plus fewer interconnections and packages translate      into greatly  improved system reliability.

• Lower Power: CMOS and fewer packages combine to reduce power consumption.
• Reduced complexity: Since PLDs consume lower power requirements less board space    simpler testing procedures .

• PLDs are field- programmable i. e., can be programmed outside of the manufacturing environment
• PLDs are erasable and reprogrammable.i.e allows updating a device or correction of errors and allows  to reuse the device for a different design – the ultimate in re-usability!

Types of Programmable Logic

Programmable logic devices are available in many different types. The current range of devices span from small devices capable of implementing only a handful of logic equations to huge FPGAs that can hold an entire processor core and peripherals. In addition to this incredible difference in size there is also much variation in architecture.

Programmable logic devices can be divided into three distinct architectural groups.

• Simple Programmable Logic Devices – SPLDs
• Complex Programmable Logic Devices – CPLDs
• Field Programmable Gate Arrays – FPGAs

Simple Programmable Devices (SPLDs):-

SPLDs are the simplest, smallest and least-expensive type of programmable logic device. These devices typically have logic gates laid out in arrays where the interconnection between these arrays are configurable by the user.

The term SPLD covers several types of device:

• Programmable Logic Array (PLA) – This device has both programmable AND and OR planes.
• Field Programmable Logic Array (FPLA) – Same as PLA but can be erased and re-programmed.
• Programmable Array Logic (PAL) – This device has a programmable AND plane and a fixed OR plane.
• GAL – This device has the same logical properties as the PAL but can be erased and reprogrammed

Figure  shows a general structure of an SPLD. The connection link across two wires can either be pre-defined or programmable depending on the type of SPLD.

Figure 1- SPLD Structure