An active edge on the write input causes any data on the data-in bus to be written into the word pointed to by the write pointer the empty output is placed in its inactive state (because the device is no longer empty) and the write pointer is incremented to point to the next empty word.ĭata can be written into the FIFO until all the words in the array contain values. The write and read pointers chase each other around the memory array in an endless loop. The initialization also causes the empty output to be placed in its active state and the full output to be placed in its inactive state. The reset input is used to initialize the device, primarily by clearing the write and read pointers such that they both point to the same memory word. 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. After loading the seed value, the feedback path is selected and the device returns to its LFSR mode of operation. When the multiplexer's data input is selected, the device functions as a standard shift register and any desired seed value may be loaded.

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).

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