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AR# 32445

11.1 Known Issue - Timing - Incorrect timing analysis associated with input clocks on BUFGMUX

Description

Keywords: timing, incorrect, clock, nets, bufgmux, set_case_analysis

The timing analysis through a BUFGMUX by Timing Analyzer should analyze the correct clock input automatically based upon the select line status. I noticed that the SDC language supports this by the command: "set_case_analysis". When is Timing Analyzer going to support this, and how do I analyze these two input clocks?

Solution

There are several options available to analyze the input clocks associated with the BUFGMUX.

Option A - The input clocks associated with the BUFGMUX are driven by a DCM/PLL/MMCM.
If you place a PERIOD constraint on the input of the DCM/PLL/MMCM, then NGDBUILD creates new PERIOD constraints based upon the configuration of the DCM/PLL/MMCM component. These new PERIOD constraints are equal in priority for analysis. Since two of these new PEIROD constraints are associated with the BUFGMUX, both PERIOD constraints will try to be analyzed. Only the second new PERIOD in the PCF will be analyzed in the timing report on the initial analysis. To analyze the first new PERIOD on the second analysis, you must change the order of these two new PERIOD constraints in the PCF; that is, cut and paste the first new PERIOD constraint and place it at the end of the PCF or after the second new PERIOD constraint.

If you place a PERIOD constraint on the inputs of the BUFGMUX and post DCM/PLL/MMCM, Xilinx recommends placing PRIORITY keywords on these two PERIOD constraints. The PRIORITY keywords ensures that the timing analysis will use the correct PERIOD constraint during the initial analysis. To analyze the other PERIOD constraint, edit the PCF, change the PRIORITY value for the PERIOD constraints, and analyze again.

Option B - The input clocks associated with the BUFGMUX are not driven by a DCM/PLL/MMCM.
If you place a PERIOD constraint on the inputs of the BUFGMUX, Xilinx recommends placing PRIORITY keywords on these two PERIOD constraints. The PRIORITY keywords ensures that the timing analysis will use the correct PERIOD constraint during the initial analysis. To analyze the other PERIOD constraint, edit the PCF, change the PRIORITY value for the PERIOD constraints, and analyze again.

Xilinx is planning to support "set_case_analysis" in the next major release of the software.
AR# 32445
Date Created 04/10/2009
Last Updated 04/22/2009
Status Active
Type General Article