Recommended Constraints Rules of IP/Sub-Module XDC - 2024.1 English

The block-level constraints must comply with the following rules:

1. Do not define clocks in the block-level constraints if they are expected to be created at the top level of the design.

   Instead they can be queried inside the block using the get_clocks -of_objects command. This command returns all the clocks that traverse a particular object in the design.

   Example:

   set blockClock [get_clocks -of_objects [get_ports clkIn]]

   If a clock needs to be defined inside the block, it must be on an input/inout port that is driving an instantiated input/inout buffer, or on the output of a cell that creates/transforms a clock (except for MMCM/PLL or special buffers that are automatically handled by the timing tools).

   Examples:

   • Input clock with input buffer
   • Clock Divider
   • GT recovered clock
2. Specify input and output delay only if the port is directly connected to the top-level port and the I/O buffer is instantiated inside the IP.

   Example:

   • Input data ports with input buffers
   • Output data ports with output buffers
3. Do not define timing exceptions between two clocks that are not bound to the IP.
4. Do not refer to clocks by name as the name may vary based on the top-level clock names or if the block is instantiated multiple times.
5. Do not add placement constraints if the block can be instantiated multiple times in a same top-level design.