diff --git a/sys/include/isrpipe/read_timeout.h b/sys/include/isrpipe/read_timeout.h
index 984de0c53f..14e13b3700 100644
--- a/sys/include/isrpipe/read_timeout.h
+++ b/sys/include/isrpipe/read_timeout.h
@@ -52,6 +52,12 @@ int isrpipe_read_timeout(isrpipe_t *isrpipe, uint8_t *buf, size_t count, uint32_
  * This function is like @ref isrpipe_read_timeout, but will only return on
  * timeout or when @p count bytes have been received.
  *
+ * A timeout only occurs if no data is received within @p timeout microseconds.
+ * Each time a byte or burst of bytes is received, the timeout is reset.
+ *
+ * @note If a timeout occurs, some bytes may have been read out of the pipe and
+ * placed into @p buf.
+ *
  * @param[in]   isrpipe    isrpipe object to operate on
  * @param[in]   buf        buffer to write to
  * @param[in]   count      number of bytes to read