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Definition: FPGA

(Field Programmable Gate Array) A chip that has its circuits programmed in the field rather than in a semiconductor facility (see fab). Containing millions of logic gates, there are a variety of FPGA architectures on the market. Compared to application specific ICs (ASICs) that are custom-designed from scratch, FPGAs have a lower gate density. See ASIC, gate array and Boolean logic.

The Difference - To Be Clear
FPGAs are a type of programmable logic chip. Whereas a regular CPU follows instructions in the software, an FPGA implements a map of connections that actually reconfigures its own hardware. A hardware circuit is always much faster when it executes the same thing as a CPU fetching and interpreting instructions to tell it what to do.

Why Use FPGAs?
When hundreds of thousands of the same chip are required, an ASIC is often designed; however, ASIC development costs millions of dollars, and FPGAs can be used to test the market and refine the design before going into full-blown ASIC production. FPGAs can also be used to deliver finished products sooner.

In addition, FPGAs are ideal for permanent installations where changing requirements demand different processing. For example, if FPGAs are used, a cell tower's upgrade from 3G to 4G can be accommodated on premesis without changing hardware.

SRAM, Flash and Antifuse Varieties
The vast majority of FPGAs use volatile static RAM (SRAM) cells to define the logic. Each time the FPGA is powered up, the SRAM is loaded with its configuration, typically from a flash memory chip. SRAM FPGAs can also be dynamically reconfigured in operation (see adaptive computing). See static RAM.

Flash memory-based FPGAs hold their content without power, but they can be reprogrammed in place. Permanently programmed antifuse-based FPGAs are popular for aerospace design because they are more radiation hardened (rad hard). See PLD and adaptive computing.

Versal System-on-Chip (FPGA and More)
With more than 35 billion transistors, Xylinx's very comprehensive Versal chip includes CPUs, RAM, I/O, adaptive hardware (FPGA part) and DSP engines (see Versal). (Image courtesy of Xilinx, www.xilinx.com)

An Excellent Resource
Everything you wanted to know about FPGAs and more is in Clive "Max" Maxfield's book. Written in Maxfield's inimitable style, which makes it thoroughly enjoyable, this book is the definitive guide to the subject. (Newnes, 2004)