Set clock skew

Hold time problems will generally occur in shift register structures or scan chains. Since by default DC treats the clock as ideal with no path delays, one must account for the network delay by using the set clock skew -propagated command. 

To specify clock delay or skew on the clock line, use the command set clock skew. The options available with this command are -delay which specifies an absolute delay, -propagated which causes DC to dynamically calculate the delay on the clock line, and the -uncertainty option which specifies the plus or minus skew possible on the clock line. By default, DC will always perform the most restrictive setup check which, in this case, is from 0 to 3. The clock waveforms must be aligned for correct setup check from 10 to 20 as shown in Figure 5.10. Another alternative, if you wish to maintain the clock waveforms is to use the following command ... 

There is no command to specify infinite drive on an internal pin. Infinite drive can only be specified on primary inputs. Use the set clock skew -delay command to specify the actual delay on the clock network. This is usually provided by the ASIC vendor. This will prevent DC from calculating the delay on the clock line, due to transition delays and loading. 

During functional mode timing analysis, there might be multi-cycle paths between the flip-flops. In the test-mode, all the flip-flops are clocked in the same cycle. Hence, perform timing analysis on the complete design after scan-insertion, without any path exceptions. Also, use the set clock skew command to account for all the delays on the different clock branches. If the clock tree is in place, use the set clock skew -propagated command. 

The set scan path command can be used to specify which scan chain the flops in a sub-design must be assigned to, depending on the clock skew on that branch. Further, this command can also be used to explicitly order scan cells within a scan chain. This command provides a means to arrange the scan-cells in the scan-chain in the order of reversed skew. 

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