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AR# 7862

12.1 Timing Constraints - How do I specify a CLOCK_TO_OUT/CLOCK_TO_PAD constraint with an internally divided/multiplied clock?

Description

How do I specify a CLOCK_TO_OUT constraint with an internally divided/multiplied clock?

Solution

Since a CLOCK_TO_OUT or a CLOCK_TO_PAD constraint must be relative to a clock pin, you must constrain relative to this clock and adjust your timing value.

For example, suppose you are using a "divide by two" clock internally, and you want to have a CLOCK_TO_OUT constraint of 10 ns. You would constrain relative to the input clock, and double the timing value to 20 ns after a clock edge.

Similarly, if you are using a multiplied clock, you would halve your desired timing value.

For additional information, see (Xilinx Answer 6905) and (Xilinx Answer 2586).

For more details on timing constraints, please see the Timing Constraints User Guide: http://www.xilinx.com/support/documentation/sw_manuals/xilinx12_1/ug612.pdf

AR# 7862
Date Created 08/21/2007
Last Updated 12/15/2012
Status Active
Type General Article