9.4 PROGRAMMABLE LOGIC
Logic devices can be classified into two broad categories: fixed and programmable. Circuits in a fixed logic device are permanent: they perform one function or set of functions, and once manufactured, they cannot be changed, as traditional ASICs can. On the other hand, programmable logic devices (PLDs) are standard, off-the-shelf parts that can be modified at any time to perform any number of functions. A key benefit of using PLDs is that, during the design phase, designers can change the circuitry as often as they want until the design operates satisfactorily. PLDs are based on rewritable memory technology: to modify the design, the device only needs to be reprogrammed. Reusability is a further attractive feature of PLDs. Many types of programmable logic devices are currently available. The range of market products includes small devices capable of implementing a handful of logic equations up to huge FPGAs that can hold an entire processor core plus a number of peripherals. Besides this impressive diversity of sizes, numerous alternative architectures are offered to the designer. Within programmable logic devices, two major types deserve to be highlighted: the complex programmable logic device (CPLD) and field programmable gate array (FPGA). They are described below.
9.4.1 Programmable Logic Devices (PLDs)
At the low end of the spectrum stand the original programmable logic devices (PLDs). They were the first chips that could be used as hardware implementation ...