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prod/doc/index.html: more fault analysis, mainly clock frequency

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Werner Almesberger 2011-05-18 07:23:11 -03:00
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@ -68,7 +68,8 @@ Defective devices can be discarded or retained for a deeper analysis.
<DD>In this context, the actual process of soldering components to
the unpopulated PCB, and all related tasks providing an input to
this process. Such related tasks include the configuration of the
SMT line, and testing and conditioning of the components to solder.
SMT line, and testing and conditioning of the components prior to
soldering.
</DL>
@ -114,12 +115,74 @@ Defective devices can be discarded or retained for a deeper analysis.
<H2>Fault analysis</H2>
<H3>Component orientation</H3>
<H3>Component placement and orientation</H3>
<H3>Supply voltages</H3>
<H3>Clock frequency</H3>
The flawless performance of the crystal oscillator is crucial for
operation. Anomalies are easy to detect with even a low-cost oscilloscope
and pinpoint specific problems and help to select further analysis steps.
<P>
The crystal used in <B>atben</B> and <B>atusb</B> has a nominal tolerance
of +/- 15 ppm at 22-28 C. Low-cost oscilloscopes typically have a timing
accuracy of
+/- 100 ppm, which means that only major excursions can be detected by
measuring the clock output with such an instrument. Full-speed USB only
requires an accuracy of +/- 2500 ppm.
We can therefore consider all results within a range of +/- 1000 ppm as
sufficient, and perform more precise measurements by other means. This
applies to <B>atben</B> as well as to <B>atusb</B>.
<P>
<H4>Measuring the clock on atben</H4>
<B>atben</B> normally does not output a clock signal. A 1 MHz clock
can be enabled with the following command:
<PRE>
atrf-txrx -d net:ben -C 1
</PRE>
This configures <B>atben</B> as a promiscuous receiver. The reception
of any IEEE 802.15.4 frame or pressing Ctrl-C will terminate the command.
<P>
<TABLE>
<TR><TH align="left">Clock<TH align="left">Action
<TR><TD>0 Hz<TD>Check voltages; check that the clock is enabled;
check for shorts around crystal; check connectivity of crystal
<TR><TD>0.999-1.001 MHz, ~3.3 Vpp<TD>Perform precision measurement with
<B>atrf-xtal</B>
<TR><TD>Other<TD>Check voltages; check for contamination around crystal
</TABLE>
<P>
<H4>Measuring the clock on atusb</H4>
The transceiver provides the clock for the microcontroller in <B>atusb</B>.
A clock signal is therefore always available. Immediately after reset,
the transceiver generates a 1 MHz clock. When the microcontrolled comes out
of reset, it raises the transceiver's clock output to 8 MHz and then
enables USB.
<P>
<TABLE>
<TR><TH align="left">Clock<TH align="left">Action
<TR><TD>0 Hz<TD>Check voltages; check for shorts around crystal; check
connectivity of crystal
<TR><TD>0.999-1.001 MHz, ~3.3 Vpp<TD>Check presence of firmware; check for
shorts on SPI signals; check connectivity of SPI signals
<TR><TD>7.992-8.008 MHz, ~3.3 Vpp<TD>Perform precision measurement with
<B>atrf-xtal</B>
<TR><TD>Other<TD>Check voltages; check for contamination around crystal
</TABLE>
<H4>Precision measurements</H4>
<P>
<HR>
Last update: 2011-05-17&nbsp;&nbsp;<I>Werner Almesberger</I>
Last update: 2011-05-18&nbsp;&nbsp;<I>Werner Almesberger</I>
<HR>
</BODY>
</HTML>