3 Replies Latest reply on Nov 5, 2015 2:39 PM by jota_1939431

    Sending data to the BCM20737S from another MCU

      Hello,

       

      I have been working with the BCM20737S to reading two pressure sensors running at 500hz.

      I now have to use two 1000hz pressure sensors. From other topics this seems like it wont be possible and will start causing trouble with Bluetooth connection etc.

       

      I am considering using a separate micro controller to read the pressure sensor, compile that data, get it in the right format, save it to memory etc.

      Then when i want to send data, communicate the data to the BCM20737s and allow this to send the data only.

       

      My question is how would you communicate between one MCU and the BCM20737s?

      Also how fast can the BCM20737s stream data (how many Mbps).

       

      Thanks,

      Luke

        • 2. Re: Sending data to the BCM20737S from another MCU

          How much and how fast you can send data will depend on your connection interval and how long it takes to transfer samples between microcontrollers.  Assuming a pretty fast connection between the BCM BLE module and the Data Acquisition Microcontroller (I recommend looking into the SPI (spiffy) interface for this...  and for iPhone you can get connection interval of 30 msec (I hear 7.5 msec is possible with other devices).  Maybe iPhone allows 20 now(?).. but from experience, with a 30 msec connection interval, if you keep the transmit queue at least half full and have a data payload of about 20 bytes per packet (23 is the max possible I think), you might be able to get 100 packets per second to the iPhone... or about 2 K Bytes per second payload transfer.  I think this is the upper limit, at least with an iPhone.  With a shorter connection interval (7.5 to 20 msec?) you might get more, but I doubt it.  I think 2K is about at the theoretical limit, although I don't really know for sure...anyway at that point you should consider standard Bluetooth instead of BLE.  The  BLE Architecture is specifically designed for the non-streaming applications.

           

          You also have to consider that if the iPhone (or other master) is busy with other tasks, it may not allow your device to have the 30 or 20 msec interval that you ask for.  You might get something less, or you might just get a disconnect if the rate your device asks for is not available or disallowed.

           

          Also, with so much activity going on and the fact that you cannot synchronize the connection events with the application thread (they run on independent clocks or schedulers).. your data acquisition activity might occasionally (or frequently) interfere with the connection activity.  There is a feature in SDK 2.2.1 that is supposed to allow your application to get an advance notification of an upcoming connection event, but after a day or two of trying I gave up and concluded that it doesn't work as advertised, or at best, it is non-trivial to get it to work..  In that case the best you can do is to try to set up your connection events and acquisition events so that they collide infrequently and when they do the collisions don't persist for longer than your slave latency allows. 

          • 3. Re: Sending data to the BCM20737S from another MCU

            Thanks Eric, great explanation!

             

            JT