Hi, I am trying to use AVRCore project from opencores.com on ACEX1K100-3 FPGA. Timing summary tells me that maximum clock is 11.xxx MHz. Since my external clock is 40MHz, I inserted a divide by 4 counter before feeding it to the CPU clock. But Quartus still says that max. clock is 11.xxx MHz. How's that, shouldn't it be now 44.xxx MHz, assuming the input counter can count to 44MHz. I am not using the input clock for any other purpose than dividing by 4. Another related question. ACEX1K datasheet says that the -3 devices do not have PLL. But Quartus always reports 0 of 1 PLL used, even when I specifically select -3 device. Which one is right, datasheet or Quartus? Thanks in advance.
Purely a point of view issue. Quartus is stating that you better not go faster than 11.xxx. If you put the divide by 4 external to the FPGA I'm sure you would be ok. If you can guarantee you seperated the clocks then I'm also sure your OK.
One way is to put the 44.xxx into one clock input and divide then that the output and wire it external to the FPGA into the other clock input.
The PLL report for the ACEX1K -3 is a bug. This device does not have a PLL, and this is a reporting bug which has been fixed in the next version of Quartus. Without seeing your design, the best I can say for the timing problem, is that you need to tell Quartus of the relationship between your
10Mhz derived clock and the 40 MHz external clock. You can do this using the Assignment Settings->Timing Requirements and Options Dialog. The steps are as follows, based on the assumption that your incoming 40 Mhz clock is clock40.
Open Assignment Settings->Timing Requirements and Options Dialog
In the Clock Settings Group select the Settings for Individual clock signals.
Click on the New button to define the new clock.
Fill in the entries in the New clock settings dialog. Set the clock settings name to clock40 (for ease of use) and the "Applies to node" should be selected to be the pin from the Node finder. The "Relationship to other clock settings" should be set to "independent of other clock settings". The fmax should be 40Mhz. Hit OK to close the New clock settings dialog box.
Next define the 11 MHz clock based on the output of the divide by 4. Click on the New button to define clock10.
Set the clock settings name to clock10. The Applies to node should be the output of the register that is the 10MHz signal. The "Relationship to other clock settings" should be set to "Based on" clock40 (defined in step4). Click on the Derived Clock Requirements, and fill in the Divide base clock field.Hit OK, and close all the dialogs.
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