Constrains for IBUFDS

[darky777]

I have a source differential clock clk125 :

  input              clk125_p_i      , 
  input              clk125_n_i      ,

in constraints it's defined as :

set_property PACKAGE_PIN F21 [get_ports "clk125_n_i"]
set_property IOSTANDARD LVDS_25 [get_ports "clk125_n_i"]
set_property PACKAGE_PIN G21 [get_ports "clk125_p_i"]
set_property IOSTANDARD LVDS_25 [get_ports "clk125_p_i"]
create_clock -period 8.000 -name clk125 [get_ports clk125_p_i]

In implementation report on timing all is ok :

Clock    Waveform(ns)     Period(ns)      Frequency(MHz)
-----    ------------     ----------      --------------
clk125   {0.000 4.000}    8.000           125.000         

This clock transfer to IBUFDS to make it single-ended clock source.But in the report, I see that the synthesizer has changed the value of the clock.

Clock                                                                                                Waveform(ns)     Period(ns)      Frequency(MHz)
-----                                                                                                ------------     ----------      --------------                  
i_wrapper/design_1_i/util_ds_buf_0/U0/IBUF_OUT[0]                                                    {0.000 5.000}      10.000          100.000         

I have tried to reassign in constrain file value of this clock but not effect.  In report i've get message like

CRITICAL WARNING: [Constraints 18-851] Could not find an automatically derived clock matching the supplied criteria for renaming. 

What is wrong?

[hongh]

Hi, @darky777 ,

It seems there is other create_clock constraint in the design also work on the port.

You can run the below command after openning the synthesized design:

write_xdc tt.xdc

In the generated tt.xdc, you can check which line of constraint conflict with your original create_clock constraint.

[darky777]

Thank you @hongh for reply.

Other constraints contain create_clock for output in IP Utility Buffer, that i've called. I've rewrited it.