SoftDevice consuming too much CPU when BLE connection poor.

[owl-wy7n]

We're using an nRF52840 with S140 SoftDevice.  It's configured as a peripheral device.  Our application uses FreeRTOS with several cooperating tasks.  When connected with the central (iPhone app), it typically uploads quite a bit of data to the central.

When BLE signals are fairly strong everything works well.

However, if signals get weak we've observed the SoftDevice consume an inordinate amount of CPU, starving important FreeRTOS tasks for many seconds.

Is there something we can do about this?  Could tweaking connection parameters help?  Which parameter settings are likely to help in this use case?

Do you have any suggestions on how to troubleshoot what is going on?

Thanks!

Bruce

[owl-wy7n]

You wrote, "...what happens ...if the buffers fill up and you are unable to send the data..."

That is very likely the problem.  I found a couple instances in our code where the send operation is immediately tried again with no delay.  That is surely the problem.  The task doing this needs to delay, giving up the CPU for a while.  Otherwise it will starve lower priority tasks.

Thanks for the help!

[ovrebekk]

Hi Bruce

This is a common situation. Essentially the stack will return NRF_ERROR_RESOURCES if the internal buffers are full, and at this point there is no point trying to send anything else until the TX event occurs (BLE_GATTS_EVT_HVN_TX_COMPLETE normally). 

There are some existing devzone tickets discussing this in more detail, such as this one. 

Best regards
Torbjørn