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Altera_Forum
Honored Contributor I
1,407 Views

PLL is not fully compensated . . .

Hi, 

 

I have a question concerning a warning that I get when compiling my design. The warning is as follows: 

 

Warning: PLL "pll_for_fft:inst28|altpll:altpll_component|pll_for_fft_altpll:auto_generated|pll1" input clock inclk[0] is not fully compensated and may have reduced jitter performance because it is fed by a non-dedicated input 

 

So as far as I understand the meaning of this warning, it says that the signal that is fed into the PLL (which in this case comes directly from a physical I/O pin of the FPGA) is not a dedicated clock signal.  

Well, in my understanding, the input pin from where that signal is routed IS a dedicated clock (input) pin. Is that so (please see attachment)? Or where can I find a dedicated clock input pin that would make this warning go away? 

 

Thanks, 

Maik
0 Kudos
6 Replies
Altera_Forum
Honored Contributor I
134 Views

 

--- Quote Start ---  

Hi, 

 

I have a question concerning a warning that I get when compiling my design. The warning is as follows: 

 

Warning: PLL "pll_for_fft:inst28|altpll:altpll_component|pll_for_fft_altpll:auto_generated|pll1" input clock inclk[0] is not fully compensated and may have reduced jitter performance because it is fed by a non-dedicated input 

 

So as far as I understand the meaning of this warning, it says that the signal that is fed into the PLL (which in this case comes directly from a physical I/O pin of the FPGA) is not a dedicated clock signal.  

Well, in my understanding, the input pin from where that signal is routed IS a dedicated clock (input) pin. Is that so (please see attachment)? Or where can I find a dedicated clock input pin that would make this warning go away? 

 

Thanks, 

Maik 

--- Quote End ---  

 

 

Does your clock pin feed the PLL directly and exclusively. I mean do you use same clock for some other sections?
Altera_Forum
Honored Contributor I
134 Views

I use the pin for another PLL . . . .

Altera_Forum
Honored Contributor I
134 Views

Hi kaz, 

 

to answer a little bit more detailed: 

 

I have this one clock in pin available and connect it directly to 2 PLLs. In my Design I have several different clock rates. So I decided to use the avilable PLLs in order to make the Timing analysis easier by just using "derive_pll_clocks -create_base_clocks" in my sdc file. 

 

Now, I see that it was not meant to conneckt more than 1 PLL to a dedicated clock input pin, is that right? 

 

So I guess I have to generate just a subset of my needed clock rates and use clock enables to get the rest of my system be clocked as needed. I cannot change the hardware anymore since this development is long finished . . . 

 

Regards, 

Maik
Altera_Forum
Honored Contributor I
134 Views

I think some clock pins can feed more than one PLL, but I don't know Cyclone III very well. Try consulting the Cyclone III device handbook, it should have a diagram showing which dedicated clock pins can feed which physical PLLs.

Altera_Forum
Honored Contributor I
134 Views

Hi aludwin, 

 

thanks, I found a table that shows how many global clock networks can be drivven by an input pin. I guess that's it. Thank you for the information. 

 

Maik
Moki
Beginner
134 Views

I had a similar problem, because my input clock was marked as a global clock.

 

Removing this from my .qsf file solved the issue:

set_instance_assignment -name GLOBAL_SIGNAL "GLOBAL CLOCK" -to clk

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