# 6

# Special Purpose Shift-Register Circuits

In Chapter 3 the properties of linear feedback shift registers used as sequence generators were examined in detail. However there are many special cases of linear and nonlinear feedback shift-register implementations that have been used in special situations. A sampling of those examples that have promise for use as stimulus generators for built-in test are described in the following.

**6.1 LINEAR FEEDBACK SHIFT REGISTERS**

The general properties of LFSRs have been discussed previously, but will be briefly reviewed here. A standard implementation was used for purposes of illustration. This is repeated below as Figure 6.1(a). In Figure 6.1(b), the LFSR implements the reciprocal characteristic polynomial. It was noted earlier that the sequences generated by the two circuits are the exact reversal of one another. Note that the sequence generated by the LFSR in Figure 6.1(a) is … 0000110101001000101111101100111 … and the sequence of Figure 6.1(b) (reciprocal polynomial) is … 0000111001101111101000100101011 …, the exact reversal, as exemplified by the reversal about the unique marker '11111'. In an LFSR having a characteristic polynomial with many terms, it is sometimes advantageous to use a different form to reduce the delay that is introduced by ...