Update -custom_registe.r

upf.md

@@ -157,7 +157,7 @@ This page describes the changes to the IEEE standard 1801 (UPF 2.1) that were ma
 <br><b>-reset</b> {<i>reset_signal [&lt;sync|async&gt; &lt;low|high&gt;]</i>}
 <br><b>-bus</b> <i>&lt;apb3 | axi4lite | avalon | wishbone&gt;</i>
 <br>[<b>-bus_param</b> {<i> name value </i>}]*
-<br>[<b>-custom_register</b> {<i> name default_value </i>}]*
+<br>[<b>-custom_register</b> {<i> name bit_width [default_value] </i>}]*
 </td>
   </tr>
   <tr>
@@ -190,8 +190,8 @@ This page describes the changes to the IEEE standard 1801 (UPF 2.1) that were ma
   <td>Interface parameters, have a look at the documentation.</td>
   </tr>
   <tr>
-  <td><b>-custom_register</b> {<i> name default_value </i>}]*</td>
-  <td>Define additional custom read-write registers, one can use for example to control event-based power mode transitioning.</td>
+  <td><b>-custom_register</b> {<i> name bit_width [default_value] </i>}]*</td>
+  <td>Define additional custom read-write registers, one can use for example to control event-based power mode transitioning. If no default_value is given, the register will be initialized with 0.</td>
   </tr>
 </table>