A programmable logic device (PLD) is an electronic component that can be configured to perform a specific logic function by the user. Unlike fixed logic devices that have predefined functions, such as logic gates or flip-flops, PLDs can be programmed and reprogrammed to implement different logic circuits. PLDs are widely used in digital systems design, as they offer flexibility, speed, and cost-effectiveness.
What is a Programmable Logic Device?
A programmable logic device is defined as an integrated circuit that contains an array of logic elements and interconnections that can be programmed by the user to implement a desired logic function. The logic elements are usually simple combinational or sequential circuits, such as AND, OR, NOT, and XOR gates, or registers. The interconnections are usually programmable switches or multiplexers that can connect the inputs and outputs of the logic elements in different ways.
The programming of a PLD can be done using a hardware description language (HDL), such as Verilog or VHDL, or a graphical user interface (GUI) software tool. The programming code or file is then downloaded to the PLD using a special device programmer or a standard interface, such as JTAG or USB. The programming code or file determines how the logic elements and interconnections are configured to perform the desired logic function.
Types of Programmable Logic Devices
There are different types of programmable logic devices, depending on their complexity, architecture, and programmability. Some of the common types are:
- Simple Programmable Logic Devices (SPLDs): These are the simplest and smallest PLDs that contain a few logic elements and interconnections. They are usually programmed using fuse or anti-fuse technology, which means that once programmed, they cannot be changed. Some examples of SPLDs are:
- Programmable Read-Only Memory (PROM): This is a memory device that can store a fixed pattern of binary data that represents a logic function. The data is programmed by burning fuses or anti-fuses in the memory cells during manufacturing. The data can be read by applying appropriate address signals to the inputs and obtaining the output signals from the outputs.
- Programmable Logic Array (PLA): This is a device that consists of two programmable arrays of AND and OR gates that can implement any sum-of-products logic expression. The AND array is programmed by burning fuses or anti-fuses in the rows that connect the inputs to the AND gates. The OR array is programmed by burning fuses or anti-fuses in the columns that connect the AND gates to the outputs.
- Programmable Array Logic (PAL): This is a device that consists of a programmable array of AND gates and a fixed array of OR gates that can implement any sum-of-products logic expression. The AND array is programmed by burning fuses or anti-fuses in the rows that connect the inputs to the AND gates. The OR array is fixed and has a predefined number of outputs.
- Generic Array Logic (GAL): This is a device that consists of a programmable array of AND gates and a fixed array of OR gates that can implement any sum-of-products logic expression. The AND array is programmed by using electrically erasable programmable read-only memory (EEPROM) cells in the rows that connect the inputs to the AND gates. The OR array is fixed and has a predefined number of outputs. The advantage of GALs over PALs is that they can be erased and reprogrammed electrically.
- Complex Programmable Logic Devices (CPLDs): These are more complex and larger PLDs that contain hundreds or thousands of logic elements and interconnections. They are usually programmed using EEPROM or flash memory technology, which means that they can be erased and reprogrammed electrically. Some examples of CPLDs are:
- Erasable Programmable Logic Device (EPLD): This is a device that consists of an array of macrocells, each containing a programmable logic block and a register. The macrocells are interconnected by a programmable switch matrix that can route signals between them. The logic block can implement any combinational or sequential logic function using EEPROM cells. The register can store one bit of data and can be configured as D-type, T-type, JK-type, or SR-type.
- Electrically Erasable Programmable Logic Device (EEPLD): This is a device that consists of an array of macrocells, each containing a programmable logic block and a register. The macrocells are interconnected by a programmable switch matrix that can route signals between them. The logic block can implement any combinational or sequential logic function using flash memory cells. The register can store one bit of data and can be configured as D-type, T-type, JK-type, or SR-type.
- Field-Programmable Gate Arrays (FPGAs): These are the most complex and largest PLDs that contain millions of logic elements and interconnections. They are usually programmed using SRAM technology, which means that they need an external memory device to store their configuration data. Some examples of FPGAs are:
- Static Random-Access Memory (SRAM) Programmed FPGA: This is a device that consists of an array of configurable logic blocks (CLBs), each containing one or more look-up tables (LUTs) and flip-flops. The CLBs are interconnected by a programmable routing network that can route signals between them. The LUTs can implement any combinational logic function using SRAM cells. The flip-flops can store one bit of data and can be configured as D-type, T-type, JK-type, or SR-type.
- Antifuse Programmed FPGA: This is a device that consists of an array of configurable logic blocks (CLBs), each containing one or more look-up tables (LUTs) and flip-flops. The CLBs are interconnected by an anti-fuse-based routing network that can route signals between them. The LUTs can implement any combinational logic function using anti-fuse cells. The flip-flops can store one bit of data and can be configured as D-type, T-type, JK-type, or SR-type.
- Flash Programmed FPGA: This is a device that consists of an array of configurable logic blocks (CLBs), each containing one or more look-up tables (LUTs) and flip-flops. The CLBs are interconnected by a flash-based routing network that can route signals between them. The LUTs can implement any combinational logic function using flash memory cells. The flip-flops can store one bit of data and can be configured as D-type, T-type, JK-type, or SR-type.
Applications and Advantages of Programmable Logic Devices
Programmable logic devices have many applications and advantages in various fields of engineering and technology. Some of them are:
- Digital Systems Design: PLDs are used to design digital systems such as microprocessors, microcontrollers, digital signal processors, embedded systems, computer peripherals, communication devices, etc. PLDs offer flexibility in designing complex logic functions with fewer hardware components and wiring than discrete logic devices.
- Rapid Prototyping: PLDs are used to prototype digital systems quickly and easily without requiring expensive fabrication processes or long development cycles. PLDs allow the testing and debugging of digital systems on-the-fly by modifying their programming code without affecting their physical structure.
- Reconfigurability: PLDs are used to reconfigure digital systems dynamically according to changing requirements or environmental conditions without requiring hardware replacement or modification. PLDs enable adaptive computing by allowing switching between different modes or functions depending on the input data or external signals.
- Performance: PLDs offer high performance in terms of speed, power consumption, reliability, and scalability than fixed logic devices. PLDs enable parallel processing by allowing multiple operations to be performed simultaneously using different parts of the device.
- Cost-effectiveness: PLDs offer cost-effectiveness in terms of design time, development cost, maintenance cost, and product life cycle than fixed logic devices. PLDs reduce design time by allowing the reuse of existing code modules and libraries for different applications. PLDs reduce development costs by allowing easy modification and correction of errors without requiring hardware changes. PLDs reduce maintenance costs by allowing easy upgradeability and compatibility with new technologies without requiring hardware changes. PLDs extend the product life cycle by allowing functionality enhancement and feature addition without requiring hardware changes.
Conclusion
Programmable logic devices are electronic components that can be configured to perform specific logic functions by the user. They offer flexibility, speed, cost-effectiveness, reconfigurability, performance, and rapid prototyping advantages over fixed logic devices. They are widely used in digital systems design for various applications. They come in different types depending on their complexity, architecture, and programmability, such as SPLDs, CPLDs, and FPGAs. They are programmed using hardware description languages or graphical user interface software tools. They have revolutionized the field of digital electronics and opened new possibilities for innovation and creativity.
