| Traditionally, digital systems have been implemented either as Application Specific Integrated Circuits (ASICs) or using standard parts such as Field Programmable Gate Arrays (FPGAs). The advantages of ASICs are a high level of customization, high density, and high performance. The main disadvantages are the high costs due to the design complexity and the number of customized masks required, yield concerns due to irregular layouts, and large design times. FPGAs, which consist of an array of programmable logic blocks (PLBs) and a fixed topology of programmable interconnect, amortize design costs over multiple applications and have significantly more regular layouts. Also, due to field programmability, design times are significantly shorter. For most applications, however, FPGAs cannot match the performance or density of ASICs.; Driven by the economics of design and manufacturing of deep sub-micron integrated circuits, an emphasis is being placed on developing new, regular circuit fabrics that: (a) simplify the design process; (b) amortize the exorbitant design cost over multiple design volumes. These fabrics must leverage the regularity of FPGAs to amortize costs, yet deliver a level of performance and density close to ASICs.; To improve performance and density, recent generations of FPGAs often employ heterogeneous logic arrays. These heterogeneous FPGAs typically fall into one of two classes. In the first class, the heterogeneity is introduced by employing two or more kinds of PLB architectures in the logic array. For example, half the PLBs in the fabric could consist of a cluster of K-input look up tables (LUTs) that can implement any function of up to K inputs, while the other half consists of PLA blocks. In the second class of heterogeneous fabrics, all the PLBs are identical; however, each individual PLB is heterogeneous in nature, with a combination of different kinds of logic elements. For example, both the Xilinx XC 4000 and Lucent ORCA devices employ an array of identical, heterogeneous PLBs, each consisting of LUTs of various sizes. More recently, complex heterogeneous PLBs have emerged which consist not only of a combination of LUTs of various sizes, but also different kinds of logic gates and MUXes, and local interconnect architectures that enable various configurations of these logic elements. (Abstract shortened by UMI.)... |
