What is the difference between FPGA and CPLD?

FPGA-Field Programmable Gate Array and CPLD-Complex Programmable Logic Device-- both are programmable logic devices made by the same companies with different characteristics.

• "A Complex Programmable Logic Device (CPLD) is a Programmable Logic Device with complexity between that of PALs (Programmable Array Logic) 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".

• This is what Wiki defines.....!!

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Architecture

• Granularity is the biggest difference between CPLD and FPGA.

• FPGA are "fine-grain" devices. That means that they contain hundreds of (up to 100000) of tiny blocks (called as LUT or CLBs etc) of logic with flip-flops, combinational logic and memories.FPGAs offer much higher complexity, up to 150,000 flip-flops and large number of gates available.

• CPLDs typically have the equivalent 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.

• CPLD are "coarse-grain" devices. They contain relatively few (a few 100's max) large blocks of logic with flip-flops and combinational logic. CPLDs based on AND-OR structure.

• CPLD's have a register with associated logic (AND/OR matrix). CPLD's are mostly implemented in control applications and FPGA's in datapath applications. Because of this course grained architecture, the timing is very fixed in CPLDs.

• FPGA are RAM based. They need to be "downloaded" (configured) at each power-up. CPLD are EEPROM based. They are active at power-up i.e. as long as they've been programmed at least once.

• FPGA needs boot ROM but CPLD does not. In some systems you might not have enough time to boot up FPGA then you need CPLD+FPGA.

• Generally, the CPLD devices are not volatile, because they contain flash or erasable ROM memory in all the cases. The FPGA are volatile in many cases and hence they need a configuration memory for working. There are some FPGAs now which are nonvolatile. This distinction is rapidly becoming less relevant, as several of the latest FPGA products also offer models with embedded configuration memory.

• 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.

• Because of coarse-grain architecture, one block of logic can hold a big equation and hence CPLD have a faster input-to-output timings than FPGA.

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Features

• FPGA have special routing resources to implement binary counters,arithmetic functions like adders, comparators and RAM. CPLD don't have special features like this.

• FPGA can contain very large digital designs, while CPLD can contain small designs only.The limited complexity (<500>

• Speed: CPLDs offer a single-chip solution with fast pin-to-pin delays, even for wide input functions. Use CPLDs for small designs, where "instant-on", fast and wide decoding, ultra-low idle power consumption, and design security are important (e.g., in battery-operated equipment).

• Security: In CPLD once programmed, the design can be locked and thus made secure. Since the configuration bitstream must be reloaded every time power is re-applied, design security in FPGA is an issue.

• Power: The high static (idle) power consumption prohibits use of CPLD in battery-operated equipment. FPGA idle power consumption is reasonably low, although it is sharply increasing in the newest families.

• Design flexibility: FPGAs offer more logic flexibility and more sophisticated system features than CPLDs: clock management, on-chip RAM, DSP functions, (multipliers), and even on-chip microprocessors and Multi-Gigabit Transceivers.These benefits and opportunities of dynamic reconfiguration, even in the end-user system, are an important advantage.

• Use FPGAs for larger and more complex designs.

Click here to read what Xilinx has to say about it.

• FPGA is suited for timing circuit becauce they have more registers , but CPLD is suited for control circuit because they have more combinational circuit. At the same time, If you synthesis the same code for FPGA for many times, you will find out that each timing report is different. But it is different in CPLD synthesis, you can get the same result.

As CPLDs and FPGAs become more advanced the differences between the two device types will continue to blur. While this trend may appear to make the two types more difficult to keep apart, the architectural advantage of CPLDs combining low cost, non-volatile configuration, and macro cells with predictable timing characteristics will likely be sufficient to maintain a product differentiation for the foreseeable future.

• There are people who discuss about this. Click here to listen them.

• Finally here is one pdf document whcih is downloadable: "Architecture of FPGAs and CPLDs: A Tutorial" Download

Hoping that information and references helps you ....... comments and further references are welcome !