diff --git a/lib/swd.h b/lib/swd.h
index a14d9ad..4476753 100644
--- a/lib/swd.h
+++ b/lib/swd.h
@@ -85,6 +85,8 @@ enum swd_activation_code_e {
 };
 
 /** setup SWD peripheral
+ *  @param[in] freq clock frequency, in Hz
+ *  @note above 50 kHz the communication still works, but the clock is not periodic anymore
  */
 void swd_setup(uint32_t freq);
 /** release peripherals used for SWD communication