So far we have seen two extremes in the design options available to a digital designer-namely standard products and mask programmable ASICs. Although mask programmable ASICs offer extremely high performance they carry a large risk in terms of time and expenditure. To provide the designer with the flexibility of both, the industry has gradually developed a class of logic that can be programmed with a personal computer in the laboratory. These devices are called field programmable logic and can be either one-time programmable (utilising small fuses)or many times programmable (using either ultraviolet erasable connections or an SRAM/MUX). Because these devices contain the extra circuitry to control interconnect and functionality this overhead results in a family which is less complex and slower than the mask programmable ASICs. However, the attraction of a much lower risk can outweigh the performance problems especially for prototyping purposes.
These field programmable logic devices are divided into two groups:
• ereidland- OR programmable architectures;
• field programmable gate arrays or FPGAS.