diff --git a/cpu/sam0_common/periph/rtc_rtt.c b/cpu/sam0_common/periph/rtc_rtt.c
index ab4777f589a5..4330cd49bd5d 100644
--- a/cpu/sam0_common/periph/rtc_rtt.c
+++ b/cpu/sam0_common/periph/rtc_rtt.c
@@ -588,10 +588,22 @@ int rtc_get_time(struct tm *time)
     return 0;
 }
 
+static void _rtc_clear_alarm(void)
+{
+    /* disable alarm interrupt */
+    RTC->MODE2.INTENCLR.reg = RTC_MODE2_INTENCLR_ALARM0;
+}
+
+void rtc_clear_alarm(void)
+{
+    _rtc_clear_alarm();
+    _pm_unblock(&_pm_alarm);
+}
+
 int rtc_set_alarm(struct tm *time, rtc_alarm_cb_t cb, void *arg)
 {
     /* prevent old alarm from ringing */
-    rtc_clear_alarm();
+    _rtc_clear_alarm();
 
     /* normalize input */
     rtc_tm_normalize(time);
@@ -600,18 +612,18 @@ int rtc_set_alarm(struct tm *time, rtc_alarm_cb_t cb, void *arg)
         (time->tm_year > (reference_year + 63))) {
         return -2;
     }
-    else {
-        RTC->MODE2.Mode2Alarm[0].ALARM.reg = RTC_MODE2_ALARM_YEAR(time->tm_year - reference_year)
-                                           | RTC_MODE2_ALARM_MONTH(time->tm_mon + 1)
-                                           | RTC_MODE2_ALARM_DAY(time->tm_mday)
-                                           | RTC_MODE2_ALARM_HOUR(time->tm_hour)
-                                           | RTC_MODE2_ALARM_MINUTE(time->tm_min)
-                                           | RTC_MODE2_ALARM_SECOND(time->tm_sec);
-        RTC->MODE2.Mode2Alarm[0].MASK.reg = RTC_MODE2_MASK_SEL(6);
-    }
 
+    /* make sure that preceding changes have been applied */
     _wait_syncbusy();
 
+    RTC->MODE2.Mode2Alarm[0].ALARM.reg = RTC_MODE2_ALARM_YEAR(time->tm_year - reference_year)
+                                       | RTC_MODE2_ALARM_MONTH(time->tm_mon + 1)
+                                       | RTC_MODE2_ALARM_DAY(time->tm_mday)
+                                       | RTC_MODE2_ALARM_HOUR(time->tm_hour)
+                                       | RTC_MODE2_ALARM_MINUTE(time->tm_min)
+                                       | RTC_MODE2_ALARM_SECOND(time->tm_sec);
+    RTC->MODE2.Mode2Alarm[0].MASK.reg = RTC_MODE2_MASK_SEL(6);
+
     /* Enable IRQ */
     alarm_cb.cb  = cb;
     alarm_cb.arg = arg;
@@ -650,14 +662,6 @@ int rtc_set_time(struct tm *time)
     return 0;
 }
 
-void rtc_clear_alarm(void)
-{
-    /* disable alarm interrupt */
-    RTC->MODE2.INTENCLR.reg = RTC_MODE2_INTENCLR_ALARM0;
-
-    _pm_unblock(&_pm_alarm);
-}
-
 void rtc_poweron(void)
 {
     _poweron();
@@ -713,18 +717,32 @@ uint32_t rtt_get_alarm(void)
     return RTC->MODE0.COMP[0].reg;
 }
 
+static void _rtt_clear_alarm(void)
+{
+    /* disable compare interrupt */
+    RTC->MODE0.INTENCLR.reg = RTC_MODE0_INTENCLR_CMP0;
+}
+
+void rtt_clear_alarm(void)
+{
+    _rtt_clear_alarm();
+    _pm_unblock(&_pm_alarm);
+}
+
 void rtt_set_alarm(uint32_t alarm, rtt_cb_t cb, void *arg)
 {
     /* disable interrupt to avoid race */
-    rtt_clear_alarm();
+    _rtt_clear_alarm();
 
     /* setup callback */
     alarm_cb.cb  = cb;
     alarm_cb.arg = arg;
 
+    /* make sure that preceding changes have been applied */
+    _wait_syncbusy();
+
     /* set COMP register */
     RTC->MODE0.COMP[0].reg = alarm;
-    _wait_syncbusy();
 
     /* enable compare interrupt and clear flag */
     RTC->MODE0.INTFLAG.reg = RTC_MODE0_INTFLAG_CMP0;
@@ -736,14 +754,6 @@ void rtt_set_alarm(uint32_t alarm, rtt_cb_t cb, void *arg)
     }
 }
 
-void rtt_clear_alarm(void)
-{
-    /* disable compare interrupt */
-    RTC->MODE0.INTENCLR.reg = RTC_MODE0_INTENCLR_CMP0;
-
-    _pm_unblock(&_pm_alarm);
-}
-
 void rtt_poweron(void)
 {
     _poweron();