Connection parameters are slow to update

[AntonHellbe]

Hi,

I'm currently trying to implement a switch between slow and fast connection parameters. For a certain amount of time during a connection the link between peripheral (nRF52832) and central (Android phone) needs to speed up. Therefor when the peripheral receives a "start packet" from the central it will request to update the connection parameters using ble_conn_params_change_conn_params, however the it takes approximately 16 seconds before the link speeds up. Is there any parameter can could be changed that might reduce the time when switching from slow connection parameters to fast?

The image below shows the current measurement from the scenario. At 36s the peripheral receives the "start packet" and between 37-51s we are struggling with slow communication, at 52+ the fast connection starts.

HW: nRF52832 PCA10040

Softdevice: v6.0

SDK: v15.0

[AntonHellbe]

I found this in the Bluetooth 5.0 Core spec:

The master should allow a minimum of 6 connection events that the slave will
be listening for before the instant occurs, considering that the slave may only
be listening once every connSlaveLatency events.

So I'm assuming this explains the 16s delay since when we are in slow mode the connection interval is set to min: 1.5s max: 2s, slave latency: 0. 6 connection events = ~12 seconds.

[Kenneth]

Sounds that you have already identified the problem.

One suggestion is that the peripheral can just send some dummy notifications very frequently after receiving the "start packet", then it will at the same time "fetch" whatever the central may want to send at every interval (though this will not solve the math behind the slow update of connection interval).