NRF_ERROR_RESOURCES error

[sjmerel]

I'm trying to send data from an IMU at about 50Hz over BLE to a smartphone. I'm finding that after some number of successful transmissions, I start to get the NRF_ERROR_RESOURCES error returned from sd_ble_gatts_hvx().

My understanding is that each buffer I try to send with sd_ble_gatts_hvx() goes into a queue until it is actually sent, and this error indicates that the queue is full.  The fact that I am able to send a bunch of buffers successfully before I start to see errors suggests to me that the throughput is too low (that is, the buffers are being sent too slowly, so the queue of unsent buffers fills up, and once it's full I start to see the errors).

My program is based off the ble_app_template example.  With the default settings I was able to send about 90 samples before I start seeing errors. Based on some questions from other people about this error I saw on this forum, I played around with the NRF_SDH_BLE_GATT_MAX_MTU_SIZE and NRF_SDH_BLE_GAP_EVENT_LENGTH values in sdk_config.h, and while this did increase the number of messages successfully sent at the beginning, I still start to see the NRF_ERROR_RESOURCES after 200 or so samples.

Is there something I can do to increase the rate at which the samples are being transmitted?  Or is there some other problem?

I'm using SDK 16.0 and SoftDevice 132.

[Simon]

Use the BLE GATT Queue library. However, there is currently a bug with this module, so you have to apply the fix give in this thread.

Best regards,

Simon

[sjmerel]

I tried this; this queue seems to fill up just as before, and then the call to add an item to the queue starts to return NRF_ERROR_NO_MEM.  Increasing the size of the queue just means the queue fills up a little later.

Which leads me to think that, as I suspected, the problem is not about the timing of the writes, but the actual throughput.  

Any other suggestions on how to get the data to send more quickly?

[sjmerel]

Also, I tried the ble_app_hrs example directly from the NRF SDK 16.0 examples on a BMD-300 evaluation board.  When I increase the rate at which data is sent from 1 Hz to 50 Hz, I see the same results (data sends fine for a little while, then periodically gets NRF_ERROR_RESOURCES).   

[sjmerel]

Finally found that changing the min_conn_interval and max_conn_interval in the ble_gap_conn_params_t struct passed to sd_ble_gap_ppcp_set solved the problem.  I had thought that NRF_SDH_BLE_GAP_EVENT_LENGTH in sdk_config.h set the connection interval, but apparently not.

Still, would love to hear some clarification of what was going on here!

[Simon]

The reason you got a faster connection was probably due to decreasing min_conn_interval since it will decrease the time between two data transfer events (given that the central chooses this value). Check out the blog BLE connection intervals and events in under 5 minutes for a better understanding of this.

The define NRF_SDH_BLE_GAP_EVENT_LENGTH is the time set aside for that given connection within a connection interval, for a deeper understanding, read this thread which explains this in detail.

Best regards,

Simon