set_input/output_delay 0 constraints?

[Adrian Holmes]

Just wondering what the significance of the constraints where an input or output delay of 0 is defined? For example:

set_false_path -to [get_ports {pps_out pps_out_en}]

set_output_delay 0.000 [get_ports {pps_out pps_out_en}]

set_false_path -from [get_ports pps_in]

set_input_delay 0.000 [get_ports pps_in]

I can't work out  what the set_input/output_delay of 0 is achieving.

Can anyone help?

[Alex Forencich]

This is merely to satiate Vivado.  Apparently adding only false path constraints on all of the non-critical inputs and outputs still results in warnings that the input/output delays are not specified.  So, you need a combination of both false path and input/output delay constraints to suppress all of the warnings.  Due to the false path constraints, the actual value of the input/output delay constraints is irrelevant.  If you know of a better way to do this, then I can certainly go update all of the constraints files again. 

Alex Forencich

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[Adrian Holmes]

Hi Alex,

Yes that makes sense, thanks for clarifying. It had been bugging me for a while!

I'm a fan of removing as many warnings as possible. Gives you a chance of spotting the ones that matter!