What is Compression?

Added: 2026-06-03

[00:00:00]So what is compression? Compression is a technique  of reducing test data volume and test application time while retaining the test coverage of your  design. Inserting the compression logic requires ATPG capabilities and compression logic to insert  on the input and output side of the design. Again it adds the decompression logic at the scan input  side and at the scan output side it adds the compressor. Now the question is why you need to  go for compression logic insertion when integrated circuit size grows. In today's scenarios, the  ATPG vector takes up lots of AT memory and test time. Along with this, there is an increase in  the number of test patterns. Newer chips often have more pins and functionality that cannot be  handled by older AT equipment which have both a limited number of pins and limited buffer memory.  Also, traditional AT tools might not have enough test pins available to take care of the current  pin equipment and test cost also increases due to all these factors. So, what is the solution? You  can use certain ways to take care of runtime and data volume like increase the number of the scan  chains but it may increase the pin needed. Another solution is to increase the frequency of shifting  patterns but it may increase test power. You can reduce the number of patterns but it impacts the  test coverage. You can try signature-based output verification and again they are hard to diagnose.  You can go for self- test logic bist but they are overhead and less control over the coverage. So  what is the best solution that you can use? Yes, by using test compression we can reduce the  AT test times and test data volume without compromising the design's test coverage. So by  using test compression you can reduce the AT test times and test data volume without compromising  the design's test coverage. Here we are showing the basic architecture of a test compression  logic. Inserting compression logic involves adding a decompressor and compressor. The tool can  insert a broadcast scan and an optional exorbased test input spreader for test data decompression.  For the test output data compression, the tool can insert an exorbased or MISRbased compressor  or a variation of these types. So this is how the cycle works. You take compressed data from  the AT tool and you can expand or decompress it in on the chip and feed it into the internal scan  channels available here. Collect the response data from the output of the internal scan channels  and compact it into an output stream. And then you can send the compressed response back to  AT for comparison to the expected responses.