PROGRAMMABLE
ASICs

There are two types of programmable ASICs: programmable logic devices (PLDs) and field-programmable gate arrays (FPGAs). The distinction between the two is blurred. The only real difference is their heritage. PLDs started as small devices that could replace a handful of TTL parts, and they have grown to look very much like their younger relations, the FPGAs. We shall group both types of programmable ASICs together as FPGAs.

An FPGA is a chip that you, as a systems designer, can program yourself. An IC foundry produces FPGAs with some connections missing. You perform design entry and simulation. Next, special software creates a string of bits describing the extra connections required to make your design—the configuration file . You then connect a computer to the chip and program the chip to make the necessary connections according to the configuration file. There is no customization of any mask level for an FPGA, allowing the FPGA to be manufactured as a standard part in high volume.

FPGAs are popular with microsystems designers because they fill a gap between TTL and PLD design and modern, complex, and often expensive ASICs. FPGAs are ideal for prototyping systems or for low-volume production. FPGA vendors do not need an IC fabrication facility to produce the chips; instead they contract IC foundries to produce their parts. Being fabless relieves the FPGA vendors of the huge burden of building and running a fabrication plant (a new submicron fab costs hundreds of millions of dollars). Instead FPGA companies put their effort into the FPGA architecture and the software, where it is much easier to make a profit than building chips. They often sell the chips through distributors, but sell design software and any necessary programming hardware directly.

All FPGAs have certain key elements in common. All FPGAs have a regular array of basic logic cells that are configured using a programming technology . The chip inputs and outputs use special I/O logic cells that are different from the basic logic cells. A programmable interconnect scheme forms the wiring between the two types of logic cells. Finally, the designer uses custom software, tailored to each programming technology and FPGA architecture, to design and implement the programmable connections. The programming technology in an FPGA determines the type of basic logic cell and the interconnect scheme. The logic cells and interconnection scheme, in turn, determine the design of the input and output circuits as well as the programming scheme.

The programming technology may or may not be permanent. You cannot undo the permanent programming in one-time programmable ( OTP ) FPGAs. Reprogrammable or erasable devices may be reused many times. We shall discuss the different programming technologies in the following sections.