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AD9361 HDL - Inserting a FFT into the block design


I am currently using the FMCOMMS3 with a VC707 carrier board and I have been able to implement and use HDL project in conjunction with the No-OS SW.  I have so far made changes to the No-OS SW and am beginning to get comfortable building my own applications here, however I would like to implement a FFT on the receive IQ samples and it seems that the best approach for this is to insert a FFT IP core into the HDL design.  Could you please advise me on the following:

1. I have attached a snip of the HDL block diagram (Image 1, unchanged from first build).  I have read in some related posts that the goal is to link the FFT block to the axi_dma, placing it before the DMA.  Could you please clarify what 'before' means?  In Image 2, which I have attached, should I connect the S_AXIS_S2MM (AXI_DMA) to the M_AXIS_DATA (FFT), circled in red?  To do this, I would have to delete the S_AXIS_S2MM connection (highlighted connection) which goes to the Ethernet block.  Should I connect the FFT S_AXIS_DATA to the Ethernet port?  Sorry if this seems obvious, I don't understand the block diagram or AXI interfaces well enough at the moment to know what I am doing.

2. I would ultimately still like to have access to the time domain samples so I am ultimately hoping that once the data is is captured from the ADC and stored into DDR, I can choose to read out and process with the FFT or simply send the time domain samples through my console as currently set up.  Should I actually be creating a new slave interface on AXI_MEM_INTERCONNECT  block and connecting the FFT to this?  If so, how would I wire the FFT to this?

  • On the current HDL release (2018_R1), data is passed as follows:

    AD9361 Interface -> FIFO -> DMA Controller -> Interconnect

    Technically you can put your IP anywhere, but the recommend place is here:

    AD9361 Interface -> FIFO -> NEW IP -> DMA Controller -> Interconnect back to PS.

    If you want to get both the pre and post FFT data just add another DMA+interconnect back to the PS for that additional datapath.


  • Hi travisfcollins,

    I'm doing a similar project using the hdl_2016_r2 release. There's no FIFO between AD9361 interface and dma controller. Can you help to clarify why the FIFO is added in 2018_r1 release?

    Many thanks.

  • Hi Phil, 

    Sorry, I'm using 2015_r1 actually. After reading the release notes (, I believe that FIFO can be removed without any impact if you don't need data loopback function.


  • I would highly recommend using the current release. It can be hard for us to answer questions about older releases, especially if the tools associated with those releases have limited support from their vendors.

    In regards to the dataflow, you can put your DSP block before or after the pack blocks if you want, but know that there are 4 samples packed into the 64 bit output of the pack blocks, so your downstream processing must understand this interface. Data from the FIFO instead is clocked out at one sample at a time.

    in regards to your IP, I would recommend reading the documentation on that core so you understand what those interfaces do. How you configure them is really up to you.


  • So your recommendation is to upgrade from hdl_2016_r2 to hdl_2018_R1? I will do that. Thanks for that advice.  

    Regarding the implementation, I have read the documentation for the FFT(v9.0), util_cpack (AD core) and axi_ad9361(AD core) and from what I can tell, the FFT will assert waitstates on the S_TREADY line while processing data and requires the Master side (FIFO) to stop sending data at this time.  It doesn't look like util_cpack, util_wfifo or the axi_ad9361 have a signal which corresponds to processing that wait state.  

    Do you have any examples at all of how you might connect the S_AXIS_TREADY signal in this chain so that the FFT buffers do not get flooded with samples and begin to drop them?

    Also, the adc_sync signal on the FIFO side at the output of the packing function (util_cpack) indicates a sample of the first input channel, so this allows a downstream block (such as the FFT) to identify what are the correct I and Q samples.  the FFT does not appear to have a way of interpreting this either.

    Any potential reference designs with a succesful implementation or advice would be very helpful.   , have you had any luck with implementing a FFT core into your block design?  If so, I'm very keen to hear how you achieved it. 

    Best Regards, 


  • Hi Phil, 

    It's still in progress. My FFT block only taps into the ADC data flow and write to a different DMA channel as I still need to keep the original captured data.

    For your question above, I guess you can use a small FIFO before the FFT block just for flow control.



  • Hi travisfcollins,

    As you has been quite  prompt and helpful with the questions :) Can I ask a question which is not quite related to this thread?

    In the release site, it says hdl_2018_r1 supports Vivado 2017.4. Is there any known issue with running hdl_2018_r1  with Vivado 2015.4? How about Vivado 2018.2? 

    Many thanks,


  • The transceiver must continuously transmit and receive data, unless in TDD mode, but I would not really recommend using this to control dataflow into your FFT block.  However, the FFT block should support a pipelined mode which will basically introduce a FIFO into the data stream and allow you to continuous process data, with a small latency hit though.


  • Hi Travis, 

    Thanks for your response and ongoing support.  After reviewing the block diagram and FFT pin options in detail, I was able to insert a FFT block in the design successfully.  

    Thanks again. 


  • Hi Phil,

    I know some years have passed by since you posted this question.

    Just like you at the time, I also want to insert a FFT IP core to compute the spectrum of the received signal and currently I'm facing the exact same obstacles you had, so I'm very curious to know how you eventually solved it.

    Like you once said, any potential reference designs with a succesful implementation or advice would be very helpful.

    Kind regards,


  • Hi  

    Could you please help us to know how to connect FFT block. I have tried connecting wfifo->util_cpack->dma(fifo to stream)->FFT IP but i am unable to fetch the FFT data.

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