diff --git a/lv_obj/lv_refr.c b/lv_obj/lv_refr.c
index 1ff45cac2..ccdbdb2a9 100644
--- a/lv_obj/lv_refr.c
+++ b/lv_obj/lv_refr.c
@@ -126,6 +126,9 @@ void lv_inv_area(const area_t * area_p)
 /**
  * Set a function to call after every refresh to announce the refresh time and the number of refreshed pixels
  * @param cb pointer to a callback function (void my_refr_cb(uint32_t time_ms, uint32_t px_num))
+ *           time_ms: refresh time in [ms]
+ *           px_num: not the drawn pixels but the number of affected pixels of the screen
+ *                   (more pixels are drawn with opacity areas)
  */
 void lv_refr_set_monitor_cb(void (*cb)(uint32_t, uint32_t))
 {