CPLDs

CPLDs, or Complex Programmable Logic Devices, are a class of programmable logic devices that integrate multiple fixed-function logic blocks, called macrocells, connected by a programmable interconnect network. Each macrocell typically contains a small combinational logic function and a flip-flop, allowing both combinational and sequential logic within a single device. Unlike larger field-programmable gate arrays (FPGAs), CPLDs rely on a relatively simple, in-chip fabric and provide deterministic, predictable timing with short interconnect paths, which makes them well-suited for glue logic and timing-critical interfaces.

Most CPLDs include non-volatile configuration memory, so they retain their programmed state without external configuration. They are usually programmed via standard programming interfaces such as JTAG or SPI, and some families support in-system programmable configuration. They typically come in compact packages with a modest number of I/O pins and interfaces; densities are measured in macrocells rather than LUT slices, and modern CPLDs can implement thousands of gates equivalent.

Architecturally, CPLDs emphasize a two-level structure: a set of macrocells arranged in banks, a global interconnect matrix, and I/O logic. The interconnect is finite and predictable, enabling fast startup and deterministic timing, which is advantageous for glue logic, simple controllers, bus arbiters, and interface decoders. Power consumption is generally lower than many FPGAs at equivalent logic content, and total cost per function can be favorable for small to medium-density designs.

Compared with FPGAs, CPLDs offer simpler design flows, shorter compile times, and lower non-recurring engineering costs for small projects, but they provide less logic density and flexibility. They are commonly used for peripheral interfacing, timing control, small state machines, and glue logic in embedded systems. Vendors include Xilinx, Intel (Altera), and Lattice Semiconductor, with several families tailored to different voltage and I/O standards.