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

prod/doc/index.html

@@ -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>
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