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BLE-STACK: SPP Server -> Client communication increased latency after ~19kB transferred

Simon Falsig
Intellectual 350 points

Part Number: BLE-STACK
Other Parts Discussed in Thread: CC2650,

I'm working on an application based on the SPP examples, where an SPP-Server (BLE peripheral, custom board based on CC2650) has to transfer a large amount of data (several MB) received on its UART, to an SPP-Client (BLE central, CC2650 Launchpad), which sends the received data out on its own UART.

The protocol used for this works by transferring about 1000 bytes at a time (SPP-Server -> SPP-Client), waits for an acknowledgement (SPP-Client -> SPP-Server), then transfers the next 1000 byte blob.

This works relatively well, except for the fact that I see the transmission latency (the time it takes for RX data on the SPP-Server to appear as TX data on the SPP-Client) heavily increasing after 19 of these packets have been sent. This can be seen on the plot below.

Plot 3 (channel 0) is the input RX data to the SPP-Server, whereas Plot 4 (channel 6) is the output TX data from the SPP-Client. As can be seen, from blob 20 and on, the latency is much larger. It increases from around 25ms for the first 19 blobs, to almost 200ms for the next blobs, and is perfectly repeatable.

I can see something similar happening in the other direction also (not as visible on the plots though), where the latency (for a much smaller packet though, ~40 bytes) goes from about 25ms to almost 100ms a bit later.

Any ideas about what could be the cause of this? Could it have to do with running low on memory, hitting an unfortunate timing between the protocol on top and the BLE connection intervals, something else?

Both devices are running BLE-Stack 2.2.1.

Thanks in advance!

  • 0
    TI__Mastermind 25035 points
    Hi,

    Are you using the latest updates that was pushed yesterday? Also, in serial_port_service.c, can you try commenting out the function for the application callback:

    // If a charactersitic value changed then callback function to notify application of change
    if ( (notifyApp != 0xFF ) && SerialPortService_AppCBs && SerialPortService_AppCBs->pfnSerialPortServiceChange )
    {
    // SerialPortService_AppCBs->pfnSerialPortServiceChange( notifyApp );
    }

    to see if that makes any difference?

    Best wishes
  • 0 in reply to Zahid Haq
    Intellectual 350 points

    Hi Zahid,

    Thanks for the suggestions!

    Regarding the updates, are you referring to the commits on ble-examples on github? Then no - as our project adds quite a bit of our own code, we can't use the SPP example directly. Do you have any parts of the commit(s) in mind that might make a difference?

    Also, I tried commenting out the line you suggested - didn't help :/