Structure for optionally cascading shift registers

摘要

A set of logic elements can be configured as a cascadable shift register. In one embodiment, a logic element for an FPGA can be configured as any one of a random access memory, a cascadable shift register and a lookup table. The data-in path to the shift register includes a cascade multiplexer for optionally forming large shift registers using multiple logic elements. Each logic element includes a plurality of memory cells which are interconnected such that the data output of each memory cell can serve as the input to the next memory cell, causing the logic element to function as a shift register. The cascade multiplexer allows the last bit of one logic element to be connected to the first bit of the next logic element, bypassing any decode logic of the lookup table. Variable tap shift registers of arbitrary length can be created by cascading lookup tables of plural logic elements in series. The lookup table decode logic plus additional multiplexers can be used to select any memory cell (not necessarily the last memory cell) of the shift register.