Complex programmable logic device

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An Altera MAX 7000-series CPLD with 2500 gates.

A complex programmable logic device (CPLD) is a programmable logic device with complexity between that of PALs and FPGAs, and architectural features of both. The building block of a CPLD is the macro cell, which contains logic implementing disjunctive normal form expressions and more specialized logic operations.

Features in common with PALs:

Non-volatile configuration memory. Unlike many FPGAs, an external configuration ROM isn't required, and the CPLD can function immediately on system start-up.
For all but the largest devices, routing constrains most logic blocks to have input and output signals connected to external pins (little opportunity for internal state storage or deeply layered logic).

Features in common with FPGAs:

Large number of gates available. CPLDs typically have the equivalent of thousands to tens of thousands of logic gates, allowing implementation of moderately complicated data processing devices. PALs typically have a few hundred gate equivalents at most, while FPGAs typically range from tens of thousands to several million.
Some provisions for logic more flexible than sum-of-product expressions, including complicated feedback paths between macro cells, and specialized logic for implementing various commonly-used functions, such as integer arithmetic.

The most noticeable difference between a large CPLD and a small FPGA is the presence of on-chip non-volatile memory in the CPLD. The differences in architectural approach become more apparent farther from this intermediate region.

This characteristic of non-volatility means that CPLDs are often used in modern digital design to perform 'boot loader' functions before handing over control to other devices not having this capability. A good example is where a CPLD is used to load configuration data for an FPGA from non-volatile memory.

CPLDs were an evolutionary step from even smaller devices that preceded them, PLAs (first shipped by Signetics), and PALs.

As FPGA's become more advanced the differences between the two device types become blurred this trend will continue until the two types are essentially indistinguishable.