Bi-directional linear feedback shift register

• US 5,790,626 A
• Filed: 09/10/1996
• Issued: 08/04/1998
• Est. Priority Date: 09/10/1996
• Status: Expired due to Term

• Current Assignee: Hewlett Packard Enterprise Development LP (Hewlett-Packard Enterprise Company)
• Original Assignee: Hewlett-Packard Company (HP Inc.)
• Inventors: Johnson, David J., Dixon, Daniel J.
• Primary Examiner(s): RUBIN, MARGARET R
• Application Number:
  US08/711,775
• Time in Patent Office: 693 Days
• Field of Search: 377/69, 377/81
• US Class Current: 377/69
• CPC Class Codes: G11C 19/00 Digital stores in which the information is moved stepwise, e.g. shift registers

Abstract

A unified bi-directional LFSR is fabricated from latches having dual (Forward and Reverse) inputs. Each such latch accepts its inputs upon receipt of a clock signal that is respectively associated with the forward or reverse direction. The appropriate collection of XOR gates exists between latch outputs and the inputs associated with a forward clock signal, so as to produce the forward sequence. Likewise, another appropriate collection of XOR gates exists between the latch outputs and the inputs associated with the reverse clock signal. To produce a "reverse" LFSR corresponding to the polynomial that is the reciprocal of the polynomial for the "forward" LFSR, the latches of the reciprocal (reverse direction) LFSR are construed as being numbered in the opposite order. That is, a single set of latches (register) has both a forward linear feedback network and a reverse linear feedback network. Only one of these two feedback networks is used at a time, but they can intermix forward and reverse steps and appear to index back and forth along the native sequence of the LFSR. That is, steps in reverse undo the most recent forward steps, and vice versa. The bi-directional LFSR is also equipped with a way to preset the latches to a preselected initial value, and it is compatible with the production of all zeros. This last feature is obtained by including an extra latch in the register (n+1)-many latches for a desired n-many bit word! and using it as a valid bit to qualify as correct the remaining latches in the register.

Claims

1. A linear feedback shift register comprising:

a collection of latches, each latch having an output, first and second control inputs, and first and second data inputs respectively active upon the presence of an associated signal at the corresponding first and second control input;

a first index signal coupled to the first control inputs of the latches in the collection thereof;

a second index signal coupled to the second control inputs of the latches in the collection thereof;

a first linear feedback network coupled to the outputs of the latches in the collection thereof, coupled to the first data inputs of the latches in the collection thereof, and producing a first sequence of values in the linear feedback shift register as instances of the first index signal are issued; and

a second linear feedback network coupled to the outputs of the latches in the collection thereof, coupled to the second data inputs of the latches in the collection thereof, and producing a second sequence of values in the linear feedback shift register as instances of the second index signal are issued.

2. A linear feedback shift register as in claim 1 wherein the second sequence of values is the exact reverse of the first sequence of values.