diff --git a/cpu/atmega_common/periph/cpuid.c b/cpu/atmega_common/periph/cpuid.c
index 9ae3673a8..c892967b0 100644
--- a/cpu/atmega_common/periph/cpuid.c
+++ b/cpu/atmega_common/periph/cpuid.c
@@ -21,6 +21,10 @@
 #include <stdint.h>
 #include "avr/boot.h"
 
+/*
+ * This uses the RC calibration byte as CPU ID, so it may not be very unique.
+ * The first three bytes are constant across different MCUs of the same series.
+ */
 void cpuid_get(void *id)
 {
     uint8_t *out = id;