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FFT Output is missing one sample when sink valid is not continuously asserted



We are using Quartus prime lite v17.1 to simulate and verify the FFT IP core.


FFT IP Config -

1) Variable streaming mode

2) Fixed point FFT

3) 64 points

4) 16 bit sink data


As per our requirement, - FFT IP Clock is 125MHz

We are providing valid sink_real, and sink_imag data at every 3rd clock of 125MHz. , so we keep SINK_VALID high for one clock and then low for next two clocks.


The output FFT shows one sample missing in every FFT frame, FFT IP provides only 63 samples every frame but SOP, EOP asserts /deasserts as per the 64 sample count.

so, EOP of first frame asserts at first sample of 2nd frame. and SOP of second frame asserts at the 2nd sample of 2nd frame.


I am also attaching the waveforms to understand the issue.


Please let us know why FFT IP core is showing such behavior.


Also note that, when SINK valid is constantly high for entire input, FFT output also comes correct with no missing sample.


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3 Replies
Hi, I would like to apologize for the delay in response. It seems like I encounter some issues with notification reaching my mailbox. Sorry for the inconvenience. As I understand it, you are observing some issues related to the FFT IP missing sample when sink valid is not continuously high. For your information, this seems to be a known issue and you may refer to the KDB as following: https://www.intel.com/content/www/us/en/programmable/support/support-resources/knowledge-base/ip/201... As a workaround, you may configure the Output Order in FFT IP to be Digit Reverse to see if works for you. Alternatively, you may need to hold the sink_valid high continously. Sorry for the inconvenience. Please let me know if there is any concern. Thank you. Best regards, Chee Pin

Hello Cpchan,


Please clarify that is this issue related to FFT IP core itself or its about quartus version issue.

As we observed this issue persists with both Quartus 17.1 (Lite edition) and Quartus 18.1 (Lite edition, standard edition).


Please also let us know, what are the other known issues with FFT IP Core.


Thanks and Regards

Tarang Jindal




Hi Tarang, Thanks for your update. For your information, this issue is related to the FFT IP instead of Quartus version. https://www.intel.com/content/www/us/en/programmable/support/support-resources/knowledge-base/kdb-fi... For other known issues with FFT IP core, it is recommended to you to visit the following KDB search link, then search "FFT" to check on the KDBs related to FFT. Please let me know if there is any concern. Thank you. Best regards, Chee Pin