Fast BLE disconnect (sd_ble_gap_disconnect)

[tmav123]

My central and peripheral are continuously connected at 1sec connection interval to conserve power on both devices.

When a button is pressed, I would like to exchange data as fast as possible.

So, I would like to change from the slow connection interval (1sec) to a faster one (20ms). To update the connection interval at such slow intervals it takes long time (more than 6sec).

Therefore, I found that it is faster to disconnect and reconnect BLE again (less than 2.5sec). But I would like to optimize this time to less than 1.5sec.

From the bellow capture, I see that the sd_ble_gap_disconnect command always takes 2 connection intervals to be executed.

Is this any reason for this (BLE specs)?

Is there any way to disconnect on the 1^st received connection event?

I am using nRF52832, S132 SD v5.1.0 & SDK v14.2.0

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[Torsten Robitzki]

Have you thought about using a smaller connection interval and a quite large slave latency value? (I don't know if and how slave latency is supported by nordic).

[tmav123]

Yes, I have tested this approach. The connection parameter update time will be reduced, but the current consumption will increase at the central side (central is battery powered too).

[Torsten Robitzki]

Yes, when the central is also power constrained, you will gain not much with slave latency. The Connection Update Procedure is limited by this requirement from the standard: "The master should allow a minimum of 6 connection events that the slave will be listening for before the instant occurs."

How much data do you have to transmit? Maybe, if you try to utilize your 1s connection intervals as much as possible (high MTU, multiple PDUs per connection event, L2CAP channels etc.), you could already achieve high throughput until your connection update procedure is done.

[tmav123]

Thanks Torsten, for your suggestion.

I don't have high amount of data to transmit, but I need to send 3-4 command/response packets. So the total response time could be around 6-8sec with 1sec connection interval.

I am already using high MTU and multiple PDUs per connection event.

[Torsten Robitzki]

One other thing that came to my mind. Maybe it is possible to create a second connection between both devices, once the button was pressed. On the first connection interval, the button pressed information could be transported, the peripheral could then start advertising very frequently and the central could then connect a second time.

[tmav123]

This is also a nice idea. The only difficulty is that the code needs to be adapted to handle both connections.

Alternatively I am thinking to reduce the connection interval to 700-800ms.

Many thanks Torsten for your help.