TOPIC : Signature Analysis

Introduction Signature analysis is a compression technique based on the concept of (CRC) Cyclic Redundancy Checking It realized in hardware using linear feedback shift registers (LFSRs). In the simplest form of this scheme, the signature generator consists of a single- input LFSR.

A type 2 LFSR used as a signature analyzer The signature is the contents of the register after the last input bit has been sampled.

Number of possible LFSR For a test bit stream of length m, there are 2 m possible response streams, one of which is correct. It will be shown later that the structure of an LFSR distributes all possible input bit streams evenly over all possible signatures, i.e., the number of bit streams that produce a specific signature is

Shift Register Polynomial Division The theory behind the use of an LFSR for signature analysis is based on the concept of polynomial division, where reminder left in the register after completion of the test process corresponds to the final signature.

Example Figure shows a single-input signature analyzer where P *(x) = 1 + x 2 + x 4 + x 5. Let the input sequence be

Example continues..

Multiple-Input Signature Registers Signature analysis can be extended to testing multiple-output circuits. A single signature analyzer could be time- multiplexed, but that would require repeating the test sequence for each output, resulting in a potentially long test time. The most common technique is to use a multiple- input signature register (MISR). Here we assume that the CUT has n (or less) outputs. It is seen that this circuit operates as n single-input signature analyzers.