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The compression ratio in DFT is basically used for TAT and TDV
TAT : Tester application Time
TDV : Test data volume. ( Size of the patterns)
It is the reduction in these two number when compared to a design which has just the scan chains and no compression techniques.
The scan compression technique available with most of the comercial tool today is to have multiple scan chains inside the core with a limited number of top level scan ports used for loading and unloading these chains. So, you require hardware in your design to support. This , there will be a decompressor to feed many internal chains from limited number of top level scan input ports and a compressor to unload the value from many internal scan chains to limited number of top scan outputs.
The TAT and TDV is achieved by lesser number of cycles needed to load the internal chains. In most curde form the compression ratio for such technique is
Hi
Thanks for reply. Synopsis tool supports the compression ratio between 10X to 50X. how to decide the compression ratio, i mean we can use 10X or 15X or 25X etc. let me know. As u said it maily depends on TAT and TDV. Is there any other factors to decide the Compression ratio?
The economics of compression is just part of the story - what you can and can't do with compression is very design dependent, and tool dependent.
Less well-behaved designs cause unknowns to be propagated through the scan chains, which require masking them in the compression circuit (usually a bunch of xors). The more masking that is done, more patterns are created. You may be better off going with a smaller ratio.
Also, at some point, just having a higher compression ratio causes the tool to generate more patterns. Think about it: you're squeezing many more internal scan chains into fewer external scan chains, and for every two that get compressed into one, you can test only one, the other gets ignored. So unless the compression tool does something extra to mitigate that, there are diminishing returns at high compression ratios. There's probably a sweet spot for each design.
Other things: how many external pins are available for use as scan pins? What is the best fit for the targeted ATE?
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