Built-in testing requires a method of checking the output responses of the circuit under test that is simpler and less storage-intensive than the conventional bit-by-bit comparison of the actual output values with the previously computed correct values. The usual method is to capture and compare some statistic, called the signature, of the circuit output responses rather than to compare the individual bits themselves. The concept is illustrated in Figure 4.1. Fault detection occurs if the signature realized by a circuit differs from the signature of a fault-free version of the circuit. This chapter provides a brief introduction to six different compression techniques: ones counting, transition counting, parity checking, cyclic codes, syndrome test, and compression using Walsh spectra.

The signature and its collection algorithm should meet four qualitative guidelines:

- The algorithm must be simple enough to be implemented as part of the built-in test circuitry.
- The implementation must be fast enough to remove it as a limiting factor in test time.
- The algorithm must provide approximately logarithmic compression of the output response data to minimize the reference-signature storage volume.
- The compression method must not lose information. Specifically ...

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