https://devzone.nordicsemi.com/f/nordic-q-a/26011/system-on-mode-in-nrf51822I&#39;m new to the nrf51822, i was connected led to one of the gpio pin of nrf51822 development board. In 1 minite i want to toggle led 20sec and remaining 40 sec in sleep. After 40 sec (sleep time) the board will wake up, then led toggle 20 sec and go toen-USTelligent Community 13Wed, 18 Oct 2017 12:00:00 GMThttps://devzone.nordicsemi.com/thread/102499?ContentTypeID=1Wed, 18 Oct 2017 12:00:00 GMT137ad170-7792-4731-bb38-c0d22fbe4515:265fd7a9-3d91-4cd2-afc5-acef9c8201f8Rajani<p>Thank&#39;s for u&#39;r answer. yes i have a reason, i want to control gpio pins of nrf51822 Dk, that&#39;s why i using external led&#39;s. even though i was not found built in led&#39;s on nrf51822 Dk.</p><div style="clear:both;"></div>https://devzone.nordicsemi.com/thread/102498?ContentTypeID=1Wed, 18 Oct 2017 11:47:19 GMT137ad170-7792-4731-bb38-c0d22fbe4515:f01d8a2e-764a-4095-b507-b373fe04034bBj&#248;rn Kvaale<p>Is there a reason you want to use an external LED &amp; not one of the built in LED&#39;s on the nrf51 dk?</p><div style="clear:both;"></div>https://devzone.nordicsemi.com/thread/102500?ContentTypeID=1Wed, 18 Oct 2017 11:46:34 GMT137ad170-7792-4731-bb38-c0d22fbe4515:92d27728-86d9-4fb8-bd4c-ffda6d9c9525Bj&#248;rn Kvaale<p>See <a href="https://devzone.nordicsemi.com/question/48648/how-to-turn-an-external-led-connected-to-nrf51822-board-from-nrf-master-control-app-in-mobile-please-tell-me-the-code/">this link</a>. As for your other question, <a href="https://devzone.nordicsemi.com/question/46936/led-flashing/">this case</a> should be helpful.</p><div style="clear:both;"></div>