AR# 37806

描述

It can be common in designs for the input clock to the DCM/PLL/MMCM to be changed during run time.

Can a mismatch with the CLKIN_PERIOD attribute cause problems?

解决方案

The CLKIN_PERIOD attribute is used by software in the following ways:

• To ensure that the DCM settings are correct (i.e., the DCM is correctly set in high or low frequency mode). This is not required for DCMs in the Spartan-3A or Spartan-6 device families, and there is no frequency mode setting for PLLs or MMCMs. If these settings are not correct, a DRC erroris seen during implementation.
• It isused by Simulation to calculate the input and output CLKs from the DCM/PLL/MMCM.

If it is required to change the CLKIN frequency to a DCM/PLL/MMCM during run time, this can be done as long as the new CLKIN still meets the requirements of the DCM/PLL/MMCM.The PFD, VCO, I/O specifications need to be met in a PLL and MMCM, while the Frequency mode, I/O specifications need to be met for a DCM.In addition, it is also important to ensure that the design meets timing with all the different frequencies applied to the DCM/PLL/MMCM. This can be done by changing the Period Constraint in the ".pcf" file and re-running Timing Analyzer; this ensures that the same implementation file is used to analyze for all CLKIN frequencies.

Important Note:If the clock input frequency is changed, the DCM/PLL/MMCM must always be reset and allowed to lock onto the new input clock. The output clocks should not be used until the LOCK output goes High.

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