Add support for custom system timer frequency

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@274 42af7a65-404d-4744-a932-0658087f49c3

Documentation/NuttxPortingGuide.html

@@ -1163,6 +1163,12 @@ The system can be re-made subsequently by just typing <code>make</code>.
     number of memory regions that the memory manager must
     handle and enables the API mm_addregion(start, end);
   </li>
+  <li>
+    <code>CONFIG_TICKS_PER_MSEC</code>: The default system timer is 100Hz
+    or <code>TICKS_PER_MSEC</code>=10.  This setting may be defined to inform NuttX
+    that the processor hardware is providing system timer interrupts at some interrupt
+    interval other than 10 msec.
+  </li>
   <li>
     <code>CONFIG_RR_INTERVAL</code>: The round robin timeslice will be set
     this number of milliseconds;  Round robin scheduling can