Digital Electronics: Principles, Devices and Applications

Programmable Logic Devices 349ClockI/OsMacrocellsand I/OpinsMacrocellsand I/OpinsI/OsI/OsI/OsMacrocellsand I/OpinsMacrocellsand I/OpinsProgrammableInterconnectMatrix (PIM)Macrocellsand I/OpinsMacrocellsand I/OpinsI/OsI/OsI/OsMacrocellsand I/OpinsMacrocellsand I/OpinsI/OsFigure 9.40FLASH-370 CPLD architecture.9.12.3 FPGAsThere are two broad categories of FPGAs, namely SRAM-based FPGAs and antifuse-based FPGAs.While Xilinx and Altera are the major players in the former category, antifuse-based devices are offeredmainly by Xilinx, Actel, Quicklogic and Cypress. FPGAs were introduced by Xilinx with the XC-2000series of devices, which have been subsequently followed up by the XC-3000 series, XC-4000 seriesand XC-5000 series of devices. Of all these, the XC-4000 series is the most widely used one. Theseare all SRAM-based. Xilinx has also introduced an antifuse-based FPGA family of FPGAs calledXC-8100.The basic architecture of the XC-4000 family is built around a two-dimensional array of configurablelogic blocks (CLBs) that can be interconnected by horizontal and vertical routing channels and aresurrounded by a perimeter of programmable input/output blocks (IOBs). CLBs provide the functionalelements for constructing the user-desired logic function, and IOBs provide the interface betweenthe package pins and internal signal lines. These devices are reconfigurable and are in-systemprogrammable. Table 9.6 gives salient features of the XC-4000X and XC-4000E series of FPGAs.Altera offers the FLEX-8000 and FLEX-10000 series of FPGAs. FLEX-8000 is SRAM-based. Itcombines the fine-grained architecture and high register count characteristics of FPGAs with the highspeed and predictable interconnect timing delays of CPLDs. The basic logic element comprises afour-input look-up table (LUT) that provides combinational capability and a programmable registerthat provides sequential capability. Table 9.7 outlines salient features of the FLEX-8000 seriesof devices.The FLEX-10000 series offers all the features of FLEX-8000 series devices, with the addition ofvariable-sized blocks of SRAM called embedded array blocks (EABs). Each of the EABs can be