Illegal paths

Illegal path violations can cause a substantial decrease in fault coverage, since the violating scan-cells will not capture data, that is, they are scan controllable only. One approach to avoid this, is to place the clocks in different capture clock groups, such that they are not clocked in the same cycle in the capture mode. Clocks are placed in 

In designs where there are multiple clock domains and datapaths between the clock domains, TC will place clocks in separate capture clock groups, provided they are independent clocks sources in the testmode and not multiplexed to one test clock. Further, the clock skew must be accounted for in the clock waveforms. 

When the test hold attribute of 1 is on the testmode port. 

In this case, TC infers only one clock clk2, and will not account for any skew on the different branches of dk2. You must perform timing analysis of the design and identify the illegal paths in the design. Shown below is the relevant portion of the check test output showing the inferred capture clock groups. 

Information Inferred capture clock group clk2. (TEST-262) 

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In this case, TC infers two clocks, dk1 and clk2, in the testmode. If the test clock waveforms are specified taking into account the skew (after checking the skew of the clocks with the ASIC vendor), TC will be able to identify the illegal paths on the datapaths and place the clocks in different capture clock groups. Hence, in Figure 8.2, if we change the waveform of the clock dk1 from the default as follows ... 

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There is one stumbling block in the path of underground chemists in 1979, the DEA made phenylacetone illegal to purchase or possess. N-methylformamide is also risky to obtain, although it is not illegal and is used in industry as a solvent. 

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