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Observer rakesh_bansal
Observer
2,076 Views
Registered: ‎06-14-2017

clarification regarding combinational flag in create_generated_clock constraint

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Hi @avrumw,

 In post :

https://forums.xilinx.com/t5/Timing-Analysis/Vivado-and-BUFGMUX-timing/td-p/444448

 

I see that you have mentioned following :

 

create_generated_clock -name clk1_post -source [get_pins BUFGMUX_inst/I0] -combinatorial [get_pins BUFGMUX_inst/O]

 

create_generated_clock -name clk2_post -source [get_pins BUFGMUX_inst/I1] -combinatorial [get_pins BUFGMUX_inst/O] -add.

 

Quick clarification needed :

 

1) I believe we need to use -master_clock option too, otherwise Vivado (v2017.2) tool throws warning that no _master_clock is defined.

2) You are using -combinational flag because generated clock (out of BUFGMUX) is travelling through combinational logic of mux, right? And that input to this logic (of MUX) is a clock.

Something like :

clockA-->Combo_logic-->clock_out.

 

In this case combo logic is mux.

a) But if we have simple AND gate of clock gating logic, then also same principle will hold true and we will need -combinational flag to create_generate_clock at output of AND gate w.r.t. the clock coming to inputs of AND gate, right?

b) What if, if the input to mux is another generated_clock based on some source clock. There also can we use -combinational flag? Circuit is something like :

source_clock-->Genereted_clock-->combo_1-->clock_out.

 

But there is an AR to put constraints :

https://www.xilinx.com/support/answers/59484.html

And there i don't see any -combinational flag in constraints. Can you please let me know what am i missing here?

 

Best Regards,

RB

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Historian
Historian
3,465 Views
Registered: ‎01-23-2009

Re: clarification regarding combinational flag in create_generated_clock constraint

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Please don't post the same question twice. I replied to the question in the other post.

 

Avrum

 

 

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Historian
Historian
3,466 Views
Registered: ‎01-23-2009

Re: clarification regarding combinational flag in create_generated_clock constraint

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Please don't post the same question twice. I replied to the question in the other post.

 

Avrum

 

 

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Observer rakesh_bansal
Observer
1,912 Views
Registered: ‎06-14-2017

Re: clarification regarding combinational flag in create_generated_clock constraint

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Hi @avrumw,

 

I created this new post because the previous post, on which i queried, was closed. And i thought you might not get notification because previous post is closed. But i think i was wrong and you got notification there.

I have updated my reply on same post (https://forums.xilinx.com/t5/Timing-Analysis/Vivado-and-BUFGMUX-timing/m-p/786960#M12166).

 

Thanks and Regards,

RB

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