Digital interrupts

slk

Hi
I am considering to use the Bela to generate audio for an EEG experiment, and for this I would like to sample data from an EEG amplifier with the Bela. The EEG amplifier transmits data via UART and have a data ready pin. I would like an interrupt on the Bela when the data ready pin changes, so I can time stamp the data in the interrupt routine, and obtain a resoultion of the time stamp which is better then what I can obtain with digital reads in the render function.
Would this be possible? and do you have a code example of how to obtain it?
Best,
Simon

giuliomoro

The highest accuracy you can get for timestamping the data is to use one of the digital inputs that are avaliable via Bela's digitalRead() functions. Those are scanned at 44.1kHz, giving you a timing resolution of 22 microseconds, which is better and more consistent than anything you could achieve by setting up an actual interrupt pin and timestamping its event.

So you'd timestamp the events from the audio thread and then erceive them via UART from a dedicated thread (see the example Communication/Serial)

slk

Thank you for a quick response.

Where would you suggest to perform the digital read - as far as I understand, the render function is called for each frame block, and from what I can see the lowest block size I can select is two frames pr. block, causing me to get a call frequency of the render function of 44.1kHz/2 = 22.05kHz -> 44 ms

On another note, I am really looking forward to the Bela Gem. I have bought a couple of Pocketbeagle 2 for testing purposes, would it be possible to start testing the Bela software on the these before the release of the the Gem?

Best,
Simon

giuliomoro

slk from what I can see the lowest block size I can select is two frames pr. block, causing me to get a call frequency of the render function of 44.1kHz/2 = 22.05kHz -> 44 ms

That's 44 microseconds, not ms. Anyhow, the accuracy of the digitalRead() is not related to the block size because it takes a frame argument, which means you can call it once per frame from the audio callback, e.g.:

uint64_t gTimestamp;
void render(BelaContext* context, void*)
{
  static uint64_t digitalCounter = 0;
  static bool pastIn = false;
  for(unsigned int n = 0; n < context->digitalFrames; ++n)
  {
    bool in = digitalRead(context, n, channel);
    if(in && !pastIn)
      gTimestamp = digitalCounter;
    pastIn = in;
    digitalCounter++;
  }
/// do audio processing here
}

slk

Sorry for the typo in the unit, but yes, 44 µs is definately acceptable.
Thank you for the code example, it is very helpful.