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