Instead, use the registers - [RTC interrupt initialization in Mode 1 without using RTCZero library](https://forum.arduino.cc/index.php?topic=492670.0) - [Getting Started with the SAM D21 Xplained Pro without ASF](https://www.digikey.com/eewiki/display/microcontroller/Getting+Started+with+the+SAM+D21+Xplained+Pro+without+ASF)
Instead, use the registers