What is minimum period check

Minimum period of the clock input of a .lib can be checked in Synopsys Primetime tool.

Minimum period of the clock input of a .lib can be checked in Synopsys Primetime tool.

Imagine a scenario where a design A, is having a clock named clk and the design is characterised with the clk’s period as a value say 2 ns, ie 500 MHz. Suppose we are reading the .lib for this design A, at the Top level for Static Timing Analysis, and the period defined for the top level clock, which is connected to A’s clk is 1 ns ie, 1GHz.

Definitely, the design A is not going to function properly. To find out such discrepancies  we can check for the min_period violations. For this the min_period constraint for the clk should be specified inside the .lib of A.

The minimum period constraint can be added in the library as below

pin (CP) {

 direction : input;

 capacitance : 1.2;

  min_period : 2; /* This is the minimum period contraint which is given inside the .lib */

This tells that the clock, which is reaching at the CP pin, should be having a minimum period of 2 ns. If it is not, then it will be shown as violated by how much slack, in the report_constraint -min_period -verbose report. This will be as well, a part in, report_constraint -all_violators report too.

The command report_analysis_coverage also covers min_period checks. It can be checked as below

report_analysis_coverage -check_type min_period

What needs to be done when min_period violations are seen ?

These violations should be checked and analysed at the initial phase of the design cycle itself.

First we need to analyse whether that particular clock is expected to reach at that design instance. Sometimes multiple clocks can reach the same pin due to some missing case analysis.

Also confirm whether the particular pin is consumed as a clock itself inside the .lib design.

After the analysis, if it turns out to be a valid min_period violation, we need to inform the top level architecture/RTL team and the design .lib owners, about the frequency discrepancy and get it fixed from them.

Reference:

  • http://tech.tdzire.com/what-is-minimum-period-check/

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