1. TOPIC : Introduction to Compression Techniques UNIT 5 : BIST and BIST Architectures Module 5.4 Compression Techniques

2. Necessity of Compression Techniques In general, after applying the test vector set to the circuit under test (CUT), the output response is compared with the correct outputs. The comparison is done bit by bit. For each vector in the test set, there is a corresponding correct output. So, there can be many outputs. These correct outputs are computed before testing and saved.

3. Contd … Since there are many test vectors, the outputs require significant memory storage. The cost of memory storage might be more than the cost of circuit. Hence, there is a necessity for another approach which requires less memory storage. Compression techniques can be used to reduce memory storage.

4. Signature (Compressed form) It is the information saved about the output response in a compressed form. The test process is carried out by comparing the already computed correct signature with the observed signature. The process of reducing the output response to a signature is known as compression.

5. Testing using Signature The testing is similar to the conventional, but here the signatures are compared instead of the outputs directly. But it comes with trade-off as signature processing costs.

6. Requirements of compression procedure The length of the signature should be logarithmic factor of length of the output, so that there is significant compression. The signatures should be different for faulty and fault- free cases to ensure no loss of information. It should not introduce any signal delays. The compression circuit should be such that it can be included in the BIST logic of the CUT.

7. Different Compression Techniques The different compression techniques that can be used are as follows: One’s count compression Transition-count compression Parity-check compression