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

Hello,

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.