CPLD is A Complex Programming Logic Device. It 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 macrocell, which contains logic implementing adjustive normal form expressions and more specialized logic operations. CPLD is a evolutionary step from even smaller devices that preceded them. The characteristic of non-volatility makes the CPLD the device of choice in modern digital designs 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.
The most distinguish difference between a large CPLD and a small FPGA is the presence of no-chip non-volatile memory in the CPLD. This distinction is rapidly becoming less relevant, as several of the latest FPGA products also offer models with embedded configuration memory.
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 many legacy CPLD devices, routing constrains most logic blocks to have input and output signals connected to external pins, reducing opportunities for internal state storage and deeply layered logic. This is usually not a factor for larger CPLDs and newer CPLD product families.
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.