Understanding programmable logic devices, requires one to understand how logic circuits work. The understanding of these basic principles enables engineers as well as hardware and software designers to implement digital logic functions that are used to program PLDs. PLDs provide flexibility, rapid prototyping, and cost savings compared to traditional methods of designing custom digital circuits.
Let us have a look at PLDs.
What is PLDs?
PLD is an abbreviation that stands for Programmable Logic Device. PLD is an electronic component that is used to build reconfigurable and programmable digital circuits. It consists of one or several programmable integrated circuit, or chip. Unlike traditional ASIC, or digital circuit chips that consist of discrete logic gates with a fixed functionality designed and programmed during functionality, PLDs are on the other hand constructed with undefined functions. PLDs can be configured and reconfigured to perform various functions based on the purpose. The ability to reprogrammed after manufacture and implement desired functions gives PLDs far vast applications compared to fixed logic devices, such as typical ASIC.
Also, unlike microprocessors and microcontrollers , a PLD changes the physical connection between the gates in the device. This makes PLDs non-computing systems but logical circuits. The simple design of PLDs offers superior and higher processing speeds than microprocessors.
How a Programmable Logic Device (PLD) Works
Programmable logic devise, PLDs are composed of a combination of several logic elements and interconnects. These logic elements are the building blocks of all operations in PLDs, they include flip flops as well as AND, OR, NOT and other gates that can be configured by users. On the other hand, interconnects are paths connecting the logic components and enabling them to work and perform complex circuitry and logical functions. The functions can be modified by the user through programs, written in dedicated languages.
Programmable Logic Devices Architecture
Programming is an essential part of PLDs as it enables users to program and configure the devices to perform specific functions or operate as specific circuitry. The logic behaviour is described using a hardware description language (HDL) such as Verilog and VHDL. To program the PLDs it is imperative to make sure that the user uses the right software and language to program a specific PLDs. The languages allow the users to describe the relationship between inputs, outputs and gates.
Although the programs are written in HDL, PLDs do not understand HDL, thus the programs have to be synthesized through a compilation layer. It converts the HDL into a lower-level representation of the logic elements and interconnects as well as optimizations for performance, power consumption and other factors. The file generated specifies how the internal resources of the PLD should be configured to implement the desired logic function. This configuration file is loaded onto the PLD using a programming tool or hardware programmer.
Languages used in Programmable Logic (HDL)
Written in a textual or symbolic style, hardware description languages (HDLs) are used to define the behaviour and connections of digital circuits. The low-complexity devices are frequently programmed using languages like PALASM, CUPL, and ABEL. For more sophisticated systems, higher-level hardware description languages like Verilog and VHDL are recommended, whereas ABEL is employed for less complex applications.
Here are a few examples of typical HDLs that are commonly used:
VHDL (VHSIC Hardware Description Language):
VHDL is a powerful and standardized HDL that allows designers to describe the behaviour and structure of digital systems. The VHSIC (Very High-Speed Integrated Circuit) programme, run by the US Department of Defence, was responsible for its creation.
Verilog:
Verilog is another widely used HDL for describing digital systems. It was developed by Phil Moorby at Gateway Design Automation, which was later acquired by Cadence Design Systems.
SystemVerilog:
Extension of Verilog, SystemVerilog offers further features for both the design and verification of digital systems. It is frequently employed in intricate digital design projects and verification settings.
AHDL (Altera Hardware Description Language):
Developed by Altera (now a division of Intel), AHDL is a proprietary HDL for defining digital systems that are intended to use their FPGA hardware.
These examples show how to utilise several HDLs to describe digital circuits and systems. Engineers and designers can select the most appropriate HDL for their needs to efficiently define and implement digital logic based on the specific project and hardware platform.
Types of Programmable Logic Devices (PLDs)
Based on the flexibility, adaptability and improving technology of the programmable logic devices, there are several types of programmable logic devices, all featuring their own characteristics. Let us have a look at these PLDs in levels of complexity. They include the following:
Programmable Logic Devices
1. Simple Programmable Logic Devices (SPLDs)
These are the fundamental PLDs, which comprise devices such as Programmable Array Logic (PAL), Programmable Logic Devices (PLDs) and Generic Array Logic (GAL). SPLDs are made up of fixed AND gates that are followed by programmable OR gates or the vice versa. They allow the user to programme the OR gate connections to generate custom logic functions.
2. Complex Programmable Logic Devices (CPLDs)
As the name suggests CPLDs, are a bit more complex than SPLDs whilst also offering more flexible. Complex Programmable Logic Arrays (CPLAs) and Complex Programmable Logic Devices (CPLDs) are examples of such devices. These devices are made up of a number of macrocells, each of which has a variety of logic circuits, flip-flops, and other programmable elements all connected by a global interconnected matrix. They can be used to create larger and more complicated logic circuits.
3. Field-Programmable Gate Arrays (FPGAs)
The most versatile type of PLD is the FPGA. They are made up of many customizable logic blocks (CLBs), programmable interconnects matrix, and I/O blocks. All these are programmable and thus blocks can be configured by users to implement various logic operations and interconnections. Basically, with FPGAs, the user can configure blocks, interconnects and I/O pins.
FPGAs are highly versatile and can be utilised for a variety of applications such as digital signal processing, hardware acceleration, and so on.
Advantages of PLDs
- Enhances rapid prototyping and easier iteration
Programmable logic devices enable engineers and designers to quickly prototype and test digital logic circuits. The process is easier to implement without designing and modifying a physical circuit. Changes to the design can be made by reprogramming the PLD, considerably accelerating the design iteration process.
Also, PLDs accelerate the development cycle by enabling swift translation of concepts into functional prototypes. This shorter time-to-market is essential in competitive industries.
- Offers flexibility and customization
Programmable Logic Devices allow designers to create custom logic functions tailored to specific project requirements. These devices offer the user more flexibility during the design stage. This is due to the fact that the designing activities include modifying the entire programming file. These modifications can be seen in the PLD's working parts and design. This flexibility is particularly valuable when designing specialized or unique logic operations.
- Cost efficient
PLDs do away with the necessity for specialised silicon fabrication, which is required for Application-Specific Integrated Circuits (ASICs). This lowers the initial costs and risks connected with chip fabrication. The ability to configure and reconfigure the design of PLDs in turn proves to be cost saving in fabrication.
- Versatility
The ability to change the device's setup makes Programmable logic devices an incredibly appealing alternative due to their customizability and personalisation.
The reduced number of integrated circuits utilised when PLDs are substituted improves circuit dependability, especially because there are fewer interconnects.
Because programmable logic devices may be erased and modified, they are suitable for situations or technology that need consistent upgrades or that must be reused at some point throughout its life cycle.
- Adaptation to Changing Requirements
PLDs can be quickly reprogrammed to accommodate changes when project needs change, eliminating the need for redesigning or remanufacturing hardware.
PLDs are highly field-programmable, which means they may be programmed totally outside of the production environment. You do not have to make the changes while building the circuit because they can be altered at a later time if necessary.
- Parallel Processing
FPGAs can perform operations in parallel, resulting in the fast and efficient execution of specified algorithms and computations.
In summary, the benefits of employing PLDs include their diversity, low cost, rapid design iteration, adaptability to changing requirements, and role in allowing efficient testing and validation. PLDs are a crucial tool in current digital design for a variety of industries and applications because of these advantages.
Final Thought
Programmable Logic Devices (PLDs) have ushered in a new era of digital logic design, allowing engineers to quickly prototype and customise logic circuits. These devices, ranging from simple SPLDs to flexible FPGAs, enable innovation across industries by reducing costs and time while increasing design flexibility. As technology advances, the potential of PLDs expands, opening up new opportunities in the digital realm.
Advanced Field-Programmable Gate Arrays (FPGAs) with enhanced integration, performance, and energy efficiency will define the future of Programmable Logic Devices (PLDs). Because of heterogeneous integration with CPUs, GPUs, and custom accelerators, FPGAs will continue to accelerate AI and machine learning applications. Specialised domain-specific PLDs will emerge to address industry-specific requirements. FPGA use will become more accessible as design tools improve. Security features will improve, promoting secure hardware enclaves. FPGAs will be used in edge computing for real-time processing, and FPGAs will be used in 5G infrastructure for flexible communication functionalities. PLDs will adapt to increasing technology demands across varied sectors in an innovative ecosystem.
