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126 lines
4.1 KiB
HTML
126 lines
4.1 KiB
HTML
<TITLE>Production and testing: Fault analysis</TITLE>
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<BODY>
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<HTML>
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<H1>Production and testing: Fault analysis</H1>
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<!-- ====================================================================== -->
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<H2>Component placement and orientation</H2>
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<!-- ====================================================================== -->
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<H2>Supply voltages</H2>
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<!-- ====================================================================== -->
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<H2>Clock frequency</H2>
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The flawless performance of the crystal oscillator is crucial for
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operation. Anomalies are easy to detect with even a low-cost oscilloscope
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and pinpoint specific problems and help to select further analysis steps.
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<P>
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The crystal used in <B>atben</B> and <B>atusb</B> has a nominal tolerance
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of +/- 15 ppm at 22-28 C. Low-cost oscilloscopes typically have a timing
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accuracy of
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+/- 100 ppm, which means that only major excursions can be detected by
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measuring the clock output with such an instrument. Full-speed USB only
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requires an accuracy of +/- 2500 ppm.
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We can therefore consider all results within a range of +/- 1000 ppm as
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sufficient, and perform more precise measurements by other means. This
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applies to <B>atben</B> as well as to <B>atusb</B>.
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<P>
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<!-- ---------------------------------------------------------------------- -->
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<H3>Measuring the clock on atben</H3>
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<B>atben</B> normally does not output a clock signal. A 1 MHz clock
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can be enabled with the following command:
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<PRE>
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atrf-txrx -d net:ben -C 1
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</PRE>
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This configures <B>atben</B> as a promiscuous receiver. The reception
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of any IEEE 802.15.4 frame or pressing Ctrl-C will terminate the command.
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<P>
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<TABLE>
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<TR><TH align="left">Clock<TH align="left">Action
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<TR><TD>0 Hz<TD>Check voltages; check that the clock is enabled;
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check for shorts around crystal; check connectivity of crystal
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<TR><TD>0.999-1.001 MHz, ~3.3 Vpp<TD>Perform precision measurement with
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<B>atrf-xtal</B>
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<TR><TD>Other<TD>Check voltages; check for contamination around crystal
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</TABLE>
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<P>
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<!-- ---------------------------------------------------------------------- -->
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<H3>Measuring the clock on atusb</H3>
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The transceiver provides the clock for the microcontroller in <B>atusb</B>.
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A clock signal is therefore always available. Immediately after reset,
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the transceiver generates a 1 MHz clock. When the microcontrolled comes out
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of reset, it raises the transceiver's clock output to 8 MHz and then
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enables USB.
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<P>
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<TABLE>
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<TR><TH align="left">Clock<TH align="left">Action
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<TR><TD>0 Hz<TD>Check voltages; check for shorts around crystal; check
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connectivity of crystal
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<TR><TD>0.999-1.001 MHz, ~3.3 Vpp<TD>Check presence of firmware; check for
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shorts on SPI signals; check connectivity of SPI signals
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<TR><TD>7.992-8.008 MHz, ~3.3 Vpp<TD>Perform precision measurement with
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<B>atrf-xtal</B>
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<TR><TD>Other<TD>Check voltages; check for contamination around crystal
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</TABLE>
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<!-- ---------------------------------------------------------------------- -->
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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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<HR>
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Up: <A href="index.html">Production and testing</A>
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Prev: <A href="test.html">Functional test</A>
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<HR>
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Last update: 2011-05-18 <I>Werner Almesberger</I>
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<HR>
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</BODY>
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</HTML>
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