The Virtex user-programmable gate array, shown in Figure 1, comprises two major configurable elements: configurable logic blocks (CLBs) and input/output blocks (IOBs). • CLBs provide the functional elements for constructing logic • IOBs provide the interface between the package pins and the CLBs CLBs interconnect through a general routing matrix (GRM). The GRM comprises an array of routing switches located at the intersections of horizontal and vertical routing channels. Each CLB nests into a VersaBlock ™ that also provides local routing resources to connect the CLB to the GRM. The VersaRing ™ I/O interface provides additional routing resources around the periphery of the device. This routing improves I/O routability and facilitates pin locking. The Virtex architecture also includes the following circuits that connect to the GRM. • Dedicated block memories of 4096 bits each • Clock DLLs for clock-distribution delay compensation and clock domain control • 3-State buffers (BUFTs) associated with each CLB that drive dedicated segmentable horizontal routing resources Values stored in static memory cells control the configurable logic elements and interconnect resources. These values load into the memory cells on power-up, and can reload if necessary to change the function of the device
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