Reponse time of WRITE WITH RESPONSE command

[Tielman]

Hi

I'm using a NRF52840 as multi central with 8 connections

SDK: 15.0; SD: 6.1 S140

My central connects to a peripheral with a connection interval of 50ms and latency of 0

When connected to 1 peripheral, from the time of issuing the sd_ble_gattc_write() command to the time I receive the BLE_GATTC_EVT_WRITE_RSP event is on average +-100ms.

As soon as I receive the response, I immediately send the next command and keep doing this.

With 2 peripherals connected, the turnaround time from command to the event is +-200ms. I tested it up to 6 connections the led to a 600ms turnaround time form sending the message to receiving the BLE_GATTC_EVT_WRITE_RSP. 

Is there an internal delay/queue/timer in the softdevice for sending messages? What else could be causing this?

Thanks

[bjorn-spockeli]

Yes, I think that you are absolutly correct. I just assumed that there were packets missing, i.e. that we didn't see the empty packets transmitted from the central in the trace.

So what I think is going on is that SoftDevice scheduler on the Central is allocating the maximum event length for each connection, i.e. connection event length == connection interval, since the NRF_SDH_BLE_GAP_EVENT_LENGTH is set to 320(400ms). If we take a look at Multilink scheduling with eight connections as a Central and minimum intervall, then we see that the minimum connection interval for C0( and other 7 connections) will be 8*connection event length, which in your case will be 8*30ms = 240ms

So setting the NRF_SDH_BLE_GAP_EVENT_LENGTH to  5, i.e. a connection event length of 6.25ms, the minimum connection interval is 5*1.25ms*8=50ms, should resolve the issue. 

Best regards 

Bjørn

[Tielman]

Hi

Great, thanks! Changing NRF_SDH_BLE_GAP_EVENT_LENGTH  to 5 does force tevent=6.25ms and the events are delivered in close succession.

Thanks! This marathon session left me with traces coming out of my ears, but I am glad it is resolved

Best Regards

Tielman

[bjorn-spockeli]

Happy to hear that you're not getting the events in close succession and I apologize that I didnt see the cause of the issue until now :)