Central connect to Peripheral without scanning

[Nick_RA]

I have an existing BLE Peripheral device based on nRF52832 which is running GATT and advertising with a long period to conserve power.

What I now need to do is use another nRF device in Central mode to connect to it, write, then disconnect after a user presses a pushbutton. To conserve power this nRF will be in deep sleep or more likely powered off between button presses.

If I know the peer address of the peripheral device is there a way to connect directly i.e. without scanning in order to reduce both power consumption and latency? The only way I've seen in the examples or docs so far is whitelist>scan>connect.

[Kazi Afroza Sultana]

Hello,

Are you using NCS or nRF5 SDK? In nR5 SDK, if central knows the MAC address of the peripheral, it is possible as long as the peripheral, you try to connect is advertising. The Softdevice will perform a scanning after the sd_ble_gap_connect() has been called regardless of the application has performed scanning or not. 

Thanks.

BR

Kazi

[Nick_RA]

nRF5 SDK, and yes the peripheral is advertising at 625ms interval.

So are you effectively saying there's no advantage, that connect is effectively the same as scanning with a whitelist?

Regards,

[Kazi Afroza Sultana]

Yes.

When we do normal scanning then connecting, we are first reading the advertisements to find a device that we want to connect. In the advertisement event what we want to connect to, we call sd_ble_gap_connect, and one of the input parameters of this function is the address, which we get from the advertising packet. Then the central will continue to scan, because it needs to send the connection packet directly after an advertising packet.

If you skip the scanning and just connect, it will be able to connect to the first advertising packet that it picks up. But scanning and then connecting takes at least two advertising packets.

[Nick_RA]

OK so it looks like in this instance I have a 1+ sec delay just to connect which I was hoping to short-circuit (a bit like ARP caching in networking parlence) but it doesn't look like I can do that. Ah well, at least I can set expectations before development so thanks.