TIP OF THE MONTH: Oct. 2007: level shifter checking

TIP OF THE MONTH: level shifter checking Here is a question that recur often with our Conformal Low Power (CLP) users: How can I disable or ignore level shifter checking on select signals that cross the voltage domain boundary? Level shifters are used in multiple supply voltage (MSV) designs to transport a data signal from one voltage value to a higher or a lower level. Since level shifters are characterized for the different voltages they operate between, the user should choose the correct level shifter cells for all appropriate voltage domain pair. A Low-to-High level shifter prevents excessive short circuit current in the CMOS device that it drives. A High-to-Lowlevel shifter is inserted to achieve safe timing closure.
Level shifters checks can be disabled by setting the appropriate switch in the run script.
For example, With the following command, any crossing from 1.2 V to 1.0 V domain will be ignored. However, a crossing from 1.2 V to 0.8 V domain will be flagged since the voltage difference between the two domains is 0.4 V, and that exceeds the tolerance value of 0.2 V specified in the command below:

set lowpower option -ignore_high_to_low 0.2

The "ignore_low_to_high" option works similarly for crossings from low to high voltage domains.

The following command only checks for the existence of a level shifter and verifies that the power connections are correct. The numeric voltage values on the nets that are connected to the level shifter are ignored:

set lowpower option -level_shifter_check connect

The warning and error messages can be filtered out using the 'add rule filter' and the 'add rule waiver' commands. These commands are not limited to level shifters and can also be used for violations reported on other low power structures:

add rule filter R1 -rule LSH1.1 -message LS_LH
   report rule check LSH1.1

Here 'LS_LH' is the textual search string, and CLP will filter out any LSH1.1 violation MESsage that contains it.
Caution: The correct combination of boolean operators should be used to prevent unintended filtering of results.
The following command will filter out only those LSH3 messages that contain the strings "u123" and "DLAT_UDP": add rule filter R2 -rule LSH3 -MES u123 -AND -MES DLAT_UDP
To remove message number 6, 10 and 12 from the LSH3 report, use the following command:

add rule waiver LSH3 6 10 12

Please see the 'Verify' manuals for more information.

Originally posted in cdnusers.org by vksingh