A programmable logic device (PLD) is an integrated circuit that a user configures to perform digital logic functions. The term covers several device families, from simple programmable logic to complex programmable logic devices (CPLDs) and field-programmable gate arrays (FPGAs); their architectures and capabilities differ.
What does PLD mean?
PLD stands for programmable logic device. NASA’s Software Engineering Handbook defines a PLD as an integrated circuit configurable by a user to implement digital logic functions. In practical terms, instead of relying only on logic functions fixed during chip manufacture, a designer configures the device to carry out the required digital operations. NASA Software Engineering Handbook, “8.30 – Flight and Ground PLD Development”.
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Which devices are considered PLDs?
PLD is an umbrella term, not one specific architecture. Commonly discussed families include simple programmable logic devices (SPLDs), CPLDs, and FPGAs. Some classifications also include programmable read-only memories (PROMs). The boundaries and labels can vary with historical context and the classification being used; Microchip, for example, groups its own electronic PLD offerings into SPLDs and CPLDs. Microchip Technology, “Programmable Logic Devices”; Microchip Technology, “SPLDs/CPLDs”.
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PAL: programmable logic for simpler functions
A programmable array logic (PAL) device uses programmable AND terms that feed fixed OR terms. PALs can replace multiple separate logic functions, a role often called glue logic because they connect or coordinate other components. Microchip Technology, “1.4 PALs,” DS20007127A.
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CPLD: more logic resources connected through interconnect
A complex programmable logic device (CPLD) has more macrocells than a PAL-style SPLD. It commonly uses a centralized interconnect bus to link logic elements and I/O, allowing more complex designs while retaining predictable timing characteristics, as described by Microchip. Documented uses include I/O expansion, memory, control, and interfacing with different memory types. Microchip Technology, “1.7 CPLDs,” DS20007127A; Microchip Technology, “SPLDs/CPLDs”.
FPGA: configurable logic with extensive routing
A field-programmable gate array (FPGA) provides configurable logic blocks and abundant routing resources. Configuration bits determine how its logic functions, and a user configures the device after manufacture. Microchip describes an FPGA as “an integrated circuit designed to be configured by you after the chip is manufactured, hence field programmable.” Microchip Technology Developer Help, “What is an FPGA?”
Depending on the device, an FPGA may also include memory, math blocks, clocking resources, specialized I/O, processors, or analog blocks. These features are device-specific rather than guaranteed by the FPGA label. Configurable logic can be used to build hardware functions such as accelerators, processors, pipelines, and peripherals. Microchip Technology Developer Help, “What is an FPGA?”
What are programmable logic devices used for?
PLDs are used when a design needs digital logic that can be configured rather than assembled solely from fixed-function components. The scale of the task helps explain why the term encompasses several architectures:
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- Glue logic and connections: PALs can combine or replace multiple discrete logic functions.
- Control and interfacing: CPLDs can support control logic, I/O expansion, memory, and interfaces between different memory types.
- Configurable hardware functions: FPGAs can be configured for functions including accelerators, processors, pipelines, and peripherals; available integrated resources depend on the particular FPGA.
How should you compare two PLDs?
The family name alone does not establish a device’s capacity or performance. When comparing specific parts, check the details that determine whether each one fits the design:
- Architecture and available logic resources
- Routing and interconnect structure
- I/O capabilities and integrated blocks
- Configuration method, including whether configuration is volatile
- Timing behavior and the intended application
These distinctions are more useful than assuming every device in one family has the same capabilities or that there is a universal numerical ranking across all PLDs.
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