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prod/doc/analysis.html: added precision clock measurement for atben

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Werner Almesberger 2011-05-18 17:49:58 -03:00
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prod/doc/analysis.html
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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>
Up: <A href="index.html">Production and testing</A>&nbsp;