diff --git a/Documentation/NuttxPortingGuide.html b/Documentation/NuttxPortingGuide.html
index 0f54a3e871d..8eb13e2970b 100644
--- a/Documentation/NuttxPortingGuide.html
+++ b/Documentation/NuttxPortingGuide.html
@@ -2220,23 +2220,23 @@ The system can be re-made subsequently by just typing <code>make</code>.
 
 <p><b>System Timer</b>
   In most implementations, system time is provided by a timer interrupt.
-  That timer interrupt runs at rate determined by <code>CONFIG_MSEC_PER_TICKS</code> (default 10 or 100Hz).
-  The timer generates an interrupt each <code>CONFIG_MSEC_PER_TICKS</code> milliseconds and increments a counter called <code>g_system_timer</code>.
-  <code>g_system_timer</code> then provides a time-base for calculating <i>up-time</i> and elapsed time intervals in units of <code>CONFIG_MSEC_PER_TICKS</code>.
+  That timer interrupt runs at rate determined by <code>CONFIG_USEC_PER_TICKS</code> (default 10000 microseconds or 100Hz.  If <code>CONFIG_SCHED_TICKLESS</code> is selected, the default is 100 microseconds).
+  The timer generates an interrupt each <code>CONFIG_USEC_PER_TICKS</code> microseconds and increments a counter called <code>g_system_timer</code>.
+  <code>g_system_timer</code> then provides a time-base for calculating <i>up-time</i> and elapsed time intervals in units of <code>CONFIG_USEC_PER_TICKS</code>.
   The range of <code>g_system_timer</code> is, by default, 32-bits.
   However, if the MCU supports type <code>long long</code> and <code>CONFIG_SYSTEM_TIME16</code> is selected,
   a 64-bit system timer will be supported instead.
 </p>
 <p><b>System Timer Accuracy</b>
-  On many system, the exact timer interval specified by <code>CONFIG_MSEC_PER_TICKS</code> cannot be achieved due to limitations in frequencies or in dividers.
-  As a result, the time interval specified by <code>CONFIG_MSEC_PER_TICKS</code> may only be approximate and there may be small errors in the apparent <i>up-time</i> time.
+  On many system, the exact timer interval specified by <code>CONFIG_USEC_PER_TICKS</code> cannot be achieved due to limitations in frequencies or in dividers.
+  As a result, the time interval specified by <code>CONFIG_USEC_PER_TICKS</code> may only be approximate and there may be small errors in the apparent <i>up-time</i> time.
   These small errors, however, will accumulate over time and after a long period of time may have an unacceptably large error in the apparent <i>up-time</i> of the MCU.
 </p>
-  If the timer tick period generated by the hardware is not exactly <code>CONFIG_MSEC_PER_TICKS</code> <i>and</i> if there you require accurate up-time for the MCU, then there are measures that you can take:
+  If the timer tick period generated by the hardware is not exactly <code>CONFIG_USEC_PER_TICKS</code> <i>and</i> if there you require accurate up-time for the MCU, then there are measures that you can take:
 </p>
 <ul>
   <li>
-    Perhaps you can adjust <code>CONFIG_MSEC_PER_TICKS</code> to a different value so that an exactly <code>CONFIG_MSEC_PER_TICKS</code> can be accomplished.
+    Perhaps you can adjust <code>CONFIG_USEC_PER_TICKS</code> to a different value so that an exactly <code>CONFIG_USEC_PER_TICKS</code> can be realized.
   </li>
   <li>
     Or you can use a technique known as <i>Delta-Sigma Modulation</i>. (Suggested by Uros Platise).  Consider the example below.
@@ -2247,7 +2247,7 @@ The system can be re-made subsequently by just typing <code>make</code>.
   Consider this case:  The system timer is a count-up timer driven at 32.768KHz.
   There are dividers that can be used, but a divider of one yields the highest accuracy.
   This counter counts up until the count equals a match value, then a timer interrupt is generated.
-  The desire frequency is 100Hz (<code>CONFIG_MSEC_PER_TICKS</code> is 10).
+  The desire frequency is 100Hz (<code>CONFIG_USEC_PER_TICKS</code> is 10000).
 </p>
 <p>
   This exact frequency of 100Hz cannot be obtained in this case.