https://devzone.nordicsemi.com/f/nordic-q-a/22841/can-nrf51-be-ble-connected-while-it-s-on-sleep-mode-connected-by-other-device-as-peripheral-modeCan nRF51 be BLE connected while it&#39;s on sleep mode? (connected by other device as Peripheral mode) I thought, to be connected as peripheral mode, It must be advertising itself to be connected, because after a little bit of Advertising, the Accepteren-USTelligent Community 13Thu, 15 Jun 2017 19:15:53 GMThttps://devzone.nordicsemi.com/thread/89803?ContentTypeID=1Thu, 15 Jun 2017 19:15:53 GMT137ad170-7792-4731-bb38-c0d22fbe4515:d06a6030-d114-4163-96cd-2517ee7bbf0eJ&#248;rgen Holmefjord<p>Hi Daniel,</p> <p>There are two power modes in the nRF51 series; system on and system off mode. You can read about the differences in <a href="https://devzone.nordicsemi.com/question/1316/what-is-the-power-and-clock-model-of-peripheral-in-nrf51/?answer=1325#post-id-1325">this thread</a>.</p> <p>When in system on mode, the chip will wakeup on any event, allowing to both advertise and keep a connection. Whenever there is something that needs to be handled, the chip will wakeup and handle the event.</p> <p>There are <strong>a lot</strong> of threads about this, please search and read some of the threads!</p> <p>Best regards,</p> <p>Jørgen</p><div style="clear:both;"></div>