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@@ -89,6 +89,31 @@ enables USB.
 
 <H3>Precision measurements</H3>
 
+The clock frequency of <B>atben</B> can be measured with an accuracy
+of about +/- 100 ppm using the program <B>atrf-xtal</B>. <B>atrf-xtal</B>
+runs directly on the Ben and measures the duration of packet transmissions.
+The transmission time depends on the bit clock which is in turn derived
+from the oscillator.
+<P>
+<B>atrf-xtal</B> requires exclusive access to the <B>atben</B>. If a
+proxy is running, it has to be removed first.
+<PRE>
+killall atrf-proxy
+atrf-xtal 100
+</PRE>
+The number reported is the number of poll loops the CPU counted. This
+value should be compared to a reference count obtained with a known to
+be good <B>atben</B> board and the same Ben at a comparable temperature.
+<P>
+<TABLE>
+<TR><TH align="left">Difference<TH align="left">Action
+<TR><TD>&gt; +/- 50 ppm<TD>Correct operation
+<TR><TD>&lt; -80 ppm<TD>Check soldering of capacitors;
+  check for contamination around crystal
+<TR><TD>&gt; +120 ppm<TD>idem
+<TR><TD>Other<TD>Divergence can be compensated by adjusting trim value
+</TABLE>
+
 <P>
 <HR>
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