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Problem with USB device audio buffer callback

Other Parts Discussed in Thread: EK-TM4C1294XL

I am trying to use the launchpad EK-TM4C1294XL as a simple USB audio class device, so that I can experiment with realtime signal processing.  I am following the guides in section 2.3 of the USB library user guide.

I believe I have followed everything correctly.  When I attach the board to my PC it shows up as a USB audio device, and I have UART logging that shows the interface becoming active, volume being set, registering the audio buffer, etc...  I also have a protocol analyser that shows audio data being streamed to the board.

However, even though I have set up an audio buffer using USBAudioBufferOut(), the callback is never called.  Nor do I get USBD_AUDIO_EVENT_DATAOUT events.  I suspect the DMA transfer is never started/finished, but I can't work out why.

I am using CCS 6, Tiva library rev 2.1.0.12573, MCU silicon is rev 1.

I am attaching a zip of my project files. 1452.audio_streamer.zip