UPGRADE YOUR BROWSER

We have detected your current browser version is not the latest one. Xilinx.com uses the latest web technologies to bring you the best online experience possible. Please upgrade to a Xilinx.com supported browser:Chrome, Firefox, Internet Explorer 11, Safari. Thank you!

cancel
Showing results for 
Search instead for 
Did you mean: 
Explorer
Explorer
288 Views
Registered: ‎06-25-2014

Clock Routing Question..

This is a sanity check question to see if what I think is actually correct. :)

My understanding is that if you did something that causes a BUFG clock to leave the dedicated clock net such as say routing the clock to an OP pin without ODDR, or using the clock as a flop IP that it will cause the entire clock net to leave the dedicated clock net. (i.e. All flops using this clock as a clock input will now be fed a clock from a non-dedicated clock net)

 

Is this correct or is the non-dedicated clock routing localized where needed? 

6 Replies
Moderator
Moderator
242 Views
Registered: ‎04-18-2011

Re: Clock Routing Question..

Interesting question. I'd expect that for a clock tree that is mainly on dedicated clock routing driving clock ports with a small excursion somewhere then the jump onto the local routing is isolated.

I suspect it depends on the device and the design in question 

-------------------------------------------------------------------------
Don’t forget to reply, kudo, and accept as solution.
-------------------------------------------------------------------------
0 Kudos
Explorer
Explorer
199 Views
Registered: ‎06-25-2014

Re: Clock Routing Question..

Thanks for the answer @klumsde ! 

 

I'm pretty sure historically I was told by Xilinx staff that the whole clock would be taken off the dedicated clock net, but as you say this could be dependent on factors such as tools/family etc. 

I was also told this quite a while back so my understanding could easily be WAY out of date. (i.e. Spartan 6 days and maybe early beta days of Vivado/Kintex where the SAE's may have still been drawing on ISE/S6 experience) 

 

This is kind of where my question is coming from where I know this information is old and am probably incorrect in my understanding. 

0 Kudos
Scholar drjohnsmith
Scholar
195 Views
Registered: ‎07-09-2009

Re: Clock Routing Question..

Would it show up on a report , the timming would be very different depending upon the solution the tools chose.

 

<== If this was helpful, please feel free to give Kudos, and close if it answers your question ==>
0 Kudos
Explorer
Explorer
184 Views
Registered: ‎06-25-2014

Re: Clock Routing Question..

@drjohnsmith 

 

Yeah, I can think of a simple test case where you say run a bit of logic (e.g. a large counter) at a high speed where it's just meeting timing and then route the clock to an output pin and see what happens. I think this would "probably" show up any differences. (I will try it when I get a spare few mins)

 

I find it interesting that this doesnt seem to be such a strait forward question. I was seriously expecting someone to post saying. "The clock does XYZ, why are you asking such a trivial question you silly person!" :)

 

 

0 Kudos
Moderator
Moderator
173 Views
Registered: ‎04-18-2011

Re: Clock Routing Question..

I would expect it to be different in ultrascale since you route to a root then distribute it out from this centre point. I am prepared to get proved wrong. 

The test i would think of is make 2 clocks come out of the MMCM one is 1/4 the rate of the other and go on BUFG. 

I would run two counters then. 

Then try store the value of the fast counter in a different register using the 1/4 clock as the CE. 

We'd be able to see what does with the routing then. 

-------------------------------------------------------------------------
Don’t forget to reply, kudo, and accept as solution.
-------------------------------------------------------------------------
Scholar drjohnsmith
Scholar
151 Views
Registered: ‎07-09-2009

Re: Clock Routing Question..

You have just proved this wonderfully

https://en.wikipedia.org/wiki/No_such_thing_as_a_stupid_question

 

its oftern the silly questons that everyone has ignored cause its silly, that turn out to be real nut crackers.

Never stop asking.

 

 

<== If this was helpful, please feel free to give Kudos, and close if it answers your question ==>
0 Kudos