Long duration of ~1.5sec to establish a connection with BLE5 on nrf52840

[MarcelR]

Hi,

we have two nrf52840 which shall connect very quickly.

The scan+advertise cycle is very fast (we have no bootloader). After switching on the peripheral it is recognized by the central in probably 0.1sec

But the connection cycle takes about 1.5sec!

Is this a limitation of the BLE technology/specification or is it possible by change our coding to establish a connection in below 100 milli seconds?
What are the parameters we should look at to speed up connection?

Thank you
Marcel

[haakonsh]

The connection is usually established very fast, what takes time is the Service Discovery procedure. In the Service discovery procedure, there are about 10-20 messages sent back and forth to iterate trough the ATTribute table of the Peripheral. If the connection interval is 100ms you will have a delay of 1-2s before the central can enable any notifications or read any characteristics. Lowering the Connection interval will reduce this delay, you can increase the connection interval to 100ms afterward if that's your goal. 

I suggest you set up a DK as a sniffer according to RF Sniffer for Bluetooth LE, and study the sniffer trace of one of your connection establishment events. This will give you a good insight into what's going on on-air.