I am getting this nasty error when synthesizing my design using ISE Studio for Spartan-6:

ERROR:Place:1108 - A clock IOB / BUFGMUX clock component pair have been found
   that are not placed at an optimal clock IOB / BUFGMUX site pair. The clock
   IOB component <PIN_ADC_CLKOUT_P> is placed at site <C17>. The corresponding
   BUFG component <adc_clkout_BUFG> is placed at site <BUFGMUX_X2Y10>. There is
   only a select set of IOBs that can use the fast path to the Clocker buffer,
   and they are not being used. You may want to analyze why this problem exists
   and correct it. If this sub optimal condition is acceptable for this design,
   you may use the CLOCK_DEDICATED_ROUTE constraint in the .ucf file to demote
   this message to a WARNING and allow your design to continue. However, the use
   of this override is highly discouraged as it may lead to very poor timing
   results. It is recommended that this error condition be corrected in the
   design. A list of all the COMP.PINs used in this clock placement rule is
   listed below. These examples can be used directly in the .ucf file to
   override this clock rule.

This particular input is an LVDS input and I have in my code:

wire adc_clkout;
IBUFDS ibuf_adc_clkout(.I(PIN_ADC_CLKOUT_P), .IB(PIN_ADC_CLKOUT_N), .O(adc_clkout));

and the ucf file reads:


There are other LVDS signals with identical definitions from whom I do not get this error.

I tried everything I imagine to get rid of this error without success. In the end I just used CLOCK_DEDICATED_ROUTE = FALSE and called it the day.

However, now I am getting timing problems when I change Verilog code that has nothing to do with the place where the error occurs. However, it has to do with this CLKOUT line. So I imagine it could have to do with this error and would like to resolve it.

However, the hardware has already been built, I can not change I/O pins!

Can I somehow get rid of this error in software?

Some remarks:

  • This is not a clock signal in the sense that it is used in multiple locations, so I would not need a clock tree etc. It is just a skew-matched clock provided by the ADC together with the serial data so I can latch the data at the rising edge of this clock.

  • The ADC clock is generated by myself within the FPGA. The PIN_ADC_CLKOUT signal is merely a skewed copy of that, skew-matched to the data lines and the skew is given by the processing of the ADC and the PCB trace roundtrip

  • This ADC clock is "duty cycled". For reference, the ADC is the LTC2323 and the timing looks like this:

  • \$\begingroup\$ This is not a minimal reproducible working example. You have connected a clock signal to a FPGA pin pair that is not meant to be used for clock signals. As you said the design is fixed, this means the PCB has a bug - as simple as it is. \$\endgroup\$
    – Paebbels
    Apr 1, 2018 at 19:47

2 Answers 2


There are a few points that need to be clarified. The signal CLKOUT is what is commonly called "data clock". The data clock is basically a copy of clock reference (SCK) aligned with the data so it can be used to sample it.

As it was correctly pointed out before, the main issue is that the data clock wasn't connected to a clock capable inputs on the device and therefore there is no optimal way to route it into the clock network.

The reason why you're getting the timing issues is using this signal as a clock - do you use it along with @posedge (or rising_edge()) statement anywhere in the code? (eventually check your synthesis netlist in PlanAhead to see if there are flip flops clocked by this signal).

To answer your question - It all depends on what you expect - for example what rate are you trying to achieve. There are several options:

  • You can resample the CLKOUT signal by faster clock and use it as "data valid" strobe for latching the SDO. Note that even this may be tough if you plan to use +100MHz clock frequencies + I'd recommend to use 2-DFF synchronizer to minimize chance of metastability issues.

  • For lower rates, use the SCK for data sampling and ignore the CLKOUT completely. There you would sample the data on the rising edge of SCK as the data are presented on the falling edge (like in SPI applications). You can use oscilloscope to ensure that the SDO transitions do not occur too close to the edge of SCK.

  • If you plan to do new PCB revision anyway, use solutions provided above as temporary workaround and connect the CLKOUT_P/N to clock capable pins in the same halfbank as the SDO


It drives register clock inputs, it is a clock signal, simple as.

Do you have the ADC source clock also connected to the FPGA on a real clock input pair? If so you might (Check the timing on the ADC most carefully) be able to ignore the ADC clock output and use a MCMM or such to produce an internal clock having the appropriate skew compared to your real clock input to latch the data in.

You could maybe get away with using this input ONLY to clock a register at the IO pains then use a mess of syncronisation flipflops to cross to a clock domain driven by a real clock input at the same rate, this would then clock all the rest of your logic.

Failing that, board respin.

  • \$\begingroup\$ The ADC clock is generated in the FPGA and also an LVDS signal (in this case I use OBUFDS). I somewhat seems to work if I use this clock directly. However, in the ADC datasheet it is strongly recommended to use the skew-matched signal of the ADC. And I find it cleaner. In any case, is the answer: No, this can not be fixed in Software? Seriously? One would need to build the entire Verilog implementation? \$\endgroup\$
    – divB
    Mar 29, 2018 at 10:08
  • \$\begingroup\$ Also, could you elaborate on your second suggestion? As I said, the ADC clock is generated by myself within the FPGA. Do you mean I could take this original signal and phase lock it to the ADC output clock? How? Also worth to say, it is not a real clock - it is duty-cycled. \$\endgroup\$
    – divB
    Mar 29, 2018 at 10:13
  • \$\begingroup\$ Can’t the requirements of these 3 lines lines be relaxed so that the CLKOUT and data lines have arbitrary delay but are just matched? (Again these lines are used in one single location). Can’t I use CLOCK_DEDICATED_ROUTE hack but still ensure that basic timing requirements are met? Can’t I force this as a non-clock line? (Why does it think it’s a clock in the first place?) My code is so tiny - given what could be done with a huge Spartan6 it’s so hard to believe that I would hit hardware limits and there would be no way to fix this in SW \$\endgroup\$
    – divB
    Mar 29, 2018 at 11:06

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