diff --git a/cpu/saml21/include/periph_cpu.h b/cpu/saml21/include/periph_cpu.h
index cd08216811..9071ea4099 100644
--- a/cpu/saml21/include/periph_cpu.h
+++ b/cpu/saml21/include/periph_cpu.h
@@ -81,6 +81,8 @@ typedef enum {
 #define RTT_CLOCK_FREQUENCY (32768U)                      /* in Hz */
 #define RTT_MIN_FREQUENCY   (RTT_CLOCK_FREQUENCY / 512U)  /* in Hz */
 #define RTT_MAX_FREQUENCY   (RTT_CLOCK_FREQUENCY)         /* in Hz */
+/* determined by tests/ztimer_underflow */
+#define RTT_MIN_OFFSET      (8U)
 /** @} */
 
 #ifdef __cplusplus