Index: 06_features/funcmap/index.html
===================================================================
--- 06_features/funcmap/index.html	(revision 9383)
+++ 06_features/funcmap/index.html	(revision 9384)
@@ -74,7 +74,43 @@
 
 <H4 autotoc="yes"> DRC: design rule checks </H4>
 <p id="drc">
-TODO
+DRC checks are ran automatically as part of the normal sch-rnd DRC process,
+unless it is disabled using the conf node <b>plugins/funcmap/drc_disable</b>.
+<p>
+The funcmap DRC check verifies that each component with an active funcmap
+has all strong function groups properly utilized. For each function (of
+a function group) there are three possible states:
+<ul>
+	<li> <i>unused</i>: the function is not assigned by any port
+	<li> <i>single</i>: the function is used by only one strong function group and is assigned to a port
+	<li> <i>multi</i>: the function is used by multiple strong function groups and is assigned to a port
+</ul>
+<p>
+A function group is valid if and only if:
+<ul>
+	<li> all of its functions are <i>unused</i> or <i>multi</i>
+	<li> all of its functions are <i>single</i> or <i>multi</i>
+</ul>
+<p>
+If a strong function group has some functions <i>unused</i> while other
+functions <i>single</i> used, that means the function group is partly
+utilized, which is an error that is reported as a DRC violation.
+<p>
+For example on the <a href="#example">attiny24</a>, there are two
+very similar sync serial devides are available with shared
+ports: SPI and USI. SPI has SCK, MISO and MOSI while USI has SCK, DI and DO.
+Both SPI and USI are strong function groups. Considerations when running the
+verification for the SPI function group:
+<ul>
+	<li> SCK is present in both SPI and USI using the same name; if it is assigned to a port, it's <i>multi</i> state, oterhwise it is in <i>unused</i>state
+	<li> If MISO is assigned but MOSI is not, that is a violation
+	<li> If MOIS is assigned but MISO is not, that is a violation
+	<li> If MISO and MISO are assigned but SCK is not, that is a violation
+	<li> If SCK is assigned but neither MISO nor MOSI, that is not reported as a violation (because SCK is <i>multi</i> so it is assumed it'd be used for another group, e.g. USI - but it is not verified while checking on SPI)
+</ul>
+<p>
+Thus either all three functions of SPI must be activated, or none of them;
+except for SCK, which may be activated (for the function group USI).
 
 <H4 autotoc="yes"> Example: using the system in practice </H4>
 <p id="example">