The write and read pointers are essentially 4-bit registers whose outputs are processed by 4:16 decoders to select one of the sixteen words in the memory array. Consider a 4-bit × 16-word first-in first-out (FIFO) memory device ( Fig 2 ).Ī brief summary of the FIFO's operation is as follows. The fact that an LFSR generates an unusual sequence of values is irrelevant in many applications. One technique for achieving this is to include a multiplexer at the input to the LFSR ( Fig 1 ). In certain applications, however, it is desirable to be able to vary the seed value. When this control signal is placed in its active state, the LFSR will load with a hard-wired seed value. A single control signal can be connected to the reset inputs on some of the registers and the set inputs on others.

One method for loading a seed value is to use registers with reset or set inputs. For this reason, it is necessary to initialize an LFSR with a seed value. Each register bit can randomly initialize containing either a logic 0 or a logic 1, and the LFSR can therefore “wake up” containing its “forbidden” value. This is of particular concern when power is first applied to the circuit. One quirk with an XOR-based LFSR is that, if it happens to find itself in the all 0s value, it will happily continue to shift all 0s indefinitely (similarly for an XNOR-based LFSR and the all 1s value). These articles are abstracted from the book Bebop to the Boolean Boogie (An Unconventional Guide to Electronics) with the kind permission of the publisher. Editors Note: This is the second in a three-part introduction to Linear Feedback Shift Registers (LFSRs).

Linear feedback shift register