title: Audio IN over USB on STM32F4
body: <p>I try to create a simple USB audio interface with audio IN and OUT on a custom board based on an <strong>STM32F412</strong>. The audio OUT (from host to target) is working, also with the help of the CubeMX setup for the audio device usb class. But somehow I can't figure out how the opposite way (from target to host) should work.</p>
  <p>I see for audio out, <code>AUDIO_PeriodicTC_FS</code> gets called periodically (every 1ms) with the <code>AUDIO_OUT_TC</code> command. It never gets called with <code>AUDIO_IN_TC</code>. I tried to call <code>HAL_PCD_EP_Transmit</code> with some audio data, but the host doesn't get the input...</p>
  <p>The descriptor should be right, at least I see both interfaces (in and out) show up on the host.</p>
  <p>Is someone experienced in this or can provide some working examples?</p>