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How to I feed a single input clock pin to 2 PLLs


Hello, I am working on a design using Arria 10 GX. I included a DDR4 EMIF IP that wants a reference clock for its PLL(s). This reference clock is placed on a clock input pin.


I also need to generate some other frequencies for totally separate clock domain. But rather than cascade the EMIF PLL, I want to just feed my input clock to both the EMIF PLL and my other PLL.


If I do this without any constraints added, the fitter complains that it cannot place one of the PLLs.


If I force my input clock signal to be a global clock, then the fitter does not complain, but I have some very strange issues later on with clock constraints that I place on the input pin not propagating to the EMIF PLL.


I will mention that this design is one that used to be in a Stratix II GX part, and the tools at the time hooked up the PLLs without any issue.


Anyone know why the tools can't hook these clocks together?

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4 Replies
Hello , As of my understanding it is not the tool issue . Actually tool is trying to help to achieve performance of the PLL within the jitter /Skew specified. You can also notice Startix II and Arria 10 are not same architecture. Few questions : Did you try to use ALTCLKCTRL IP before connect to input of the other PLL reference clock? Can I also know PLL bank that you are trying to use? How do you check there is not input to EMIF PLL when you connect the other PLL to same clock ? is that using signal tap ? Thank you , Regards, Sree


Thank you for answering. One of the first things I tried is to instantiate an ALTCLKCTRL. This had no effect, it was still unable to place my PLL. IIRC, without constraining the input to be a global or a regional clock, the tool said it could not place the PLL in 8 places that it tried. I can get the exact error message if it is helpful.


I have placed the EMIF in banks in 3A, 3B and 3C. The input clock pin is in bank 3B, pins Y6/Y7.


The EMIF pll is placec at IOPLL_3B and I believe it duplicates this in 3A and 3C. My pll (when I constrain the input to be a global clock) gets placed in IOPLL_2L.


Even though my input signal is placed where the EMIF wants it (it chose the pin locations), I get warnings from the Timing Analyzer that it cannot determine the master clock for the EMIF pll. It says candidates are inclk_133 (my signal name for the input clock) and emif_0_ref_clk (the input reference to the EMIF pll) which is fed from inclk_133.


As a quick experiment, I removed the connection to my pll and instead fed its reference from an output from the EMIF pll. So, now inclk_133 feeds the EMIF pll, the EMIF pll outputs a clock of same frequency and that feeds 2nd pll (whose outputs are unrelated and in another clock domain) as well as the rest of the FPGA fabric. I get the same exact warning from the Timing Analyzer that it cannot determine the master clock for the EMIF pll with the same 2 candidates. The result of this tells me that my timing analyzer problems are not a result of connecting the 2 plls to the same input clock but rather a problem with the EMIF IP does not like my input clock. Maybe it wants it to have a particular name...


So how to I tell the Timing Analyzer to use one of those clocks for the master? The sdc file for the EMIF is auto-generated and does not look like I should mess with it even if I could decipher it:-).







Hello Bill,

Apologize for the delay in response ; Actually i missed your updated post ,

As you mentioned , you can create the one more sdc constraint file and add to the project and specific the clock input in the sdc. It just need to match the signal name and sdc port name.


Thank you ,





Sree, Thank you for responding, even if it is 2 months late. I don’t think you understand the question. Let me clarify: I have instantiated an EMIF IP for DDR4 memory. In that IP is an IOPLL and that expects a reference clock on a pin. I also have an additional IOPLL in the FPGA. I would like to connect its reference clock input to the same pin, but when I do the fitter gets an error. Note that the EMIF actually replicates its IOPLL in all the banks used by the DDR4 which in my case is 3 banks. I have found that I can force the signal on the reference clock pin to be a global clock, and then I am able to avoid the fitter error. It appears there is something special about how the tools want to hook up the reference clock to the EMIF PLL and it does not want the clock to go anywhere else. My question is, what is the best way to hook 2 PLLs up? One possibility is that for an EMIF IP, you should never connect its reference clock to anything else. Another possibility is that you need to do XYZ in order for the fitter to work. If the answer is XYZ, please tell me what XYZ is. I am just trying to understand why things are the way they are. This is not documented anywhere that I can find. Bill