diff --git a/drivers/rtt_rtc/rtt_rtc.c b/drivers/rtt_rtc/rtt_rtc.c
index e70fd9691a..0de3fe296c 100644
--- a/drivers/rtt_rtc/rtt_rtc.c
+++ b/drivers/rtt_rtc/rtt_rtc.c
@@ -26,6 +26,7 @@
 #include <stdlib.h>
 #include <string.h>
 
+#include "irq.h"
 #include "periph/rtc.h"
 #include "periph/rtt.h"
 #include "timex.h"
@@ -138,16 +139,16 @@ int rtc_set_time(struct tm *time)
 
 int rtc_get_time_ms(struct tm *time, uint16_t *ms)
 {
-    uint32_t tmp, prev = rtc_now;
+    uint32_t now, tmp;
+    unsigned state = irq_disable();
 
-    /* repeat calculation if an alarm triggered in between */
-    do {
-        uint32_t now = rtt_get_counter();
-        tmp = _rtc_now(now);
+    now  = rtt_get_counter();
+    tmp  = _rtc_now(now);
 
-        *ms = (SUBSECONDS(now - last_alarm) * MS_PER_SEC) / RTT_SECOND;
-    } while (prev != rtc_now);
+    *ms = (SUBSECONDS(now - last_alarm) * MS_PER_SEC)
+        / RTT_SECOND;
 
+    irq_restore(state);
     rtc_localtime(tmp, time);
 
     return 0;
@@ -155,15 +156,14 @@ int rtc_get_time_ms(struct tm *time, uint16_t *ms)
 
 int rtc_get_time(struct tm *time)
 {
-    uint32_t prev = rtc_now;
+    uint32_t now, tmp;
+    unsigned state = irq_disable();
 
-    /* repeat calculation if an alarm triggered in between */
-    do {
-        uint32_t now = rtt_get_counter();
-        uint32_t tmp = _rtc_now(now);
+    now  = rtt_get_counter();
+    tmp  = _rtc_now(now);
 
-        rtc_localtime(tmp, time);
-    } while (prev != rtc_now);
+    irq_restore(state);
+    rtc_localtime(tmp, time);
 
     return 0;
 }
@@ -186,6 +186,7 @@ int rtc_set_alarm(struct tm *time, rtc_alarm_cb_t cb, void *arg)
     alarm_cb_arg = arg;
     alarm_cb     = cb;
 
+    /* RTT interrupt is disabled here */
     rtc_now      = _rtc_now(now);
     _update_alarm(now);