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git://projects.qi-hardware.com/ben-wpan.git
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340 lines
11 KiB
Plaintext
340 lines
11 KiB
Plaintext
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML>
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<TITLE>Production and testing: Fault analysis</TITLE>
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<BODY bgcolor="#ffffff" link="#000000" vlink="#404040">
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<INCLUDE file="style.inc">
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<PAGE_BAR title="Production and testing">
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<PAGE_ITEM href="setup.html">Software setup</PAGE_ITEM>
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<PAGE_ITEM href="flash.html">Flashing</PAGE_ITEM>
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<PAGE_ITEM href="test.html">Functional test</PAGE_ITEM>
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<PAGE_CURR href="analysis.html">Fault analysis</PAGE_CURR>
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</PAGE_BAR>
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<SECTION_BAR>
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<SECTION_ITEM href="#orientation">Component orientation</SECTION_ITEM>
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<SECTION_ITEM href="#voltages">Voltages</SECTION_ITEM>
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<SECTION_ITEM href="#clock">Clock frequency</SECTION_ITEM>
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</SECTION_BAR>
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<!-- ====================================================================== -->
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<SECTION ref="orientation" title="Component orientation">
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There are several components whose footprint has a 180° or even 90°
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rotational symmetry, but the component's electrical function is not symmetric.
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Therefore, if such a component is incorrectly rotated, it will appear to the
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casual observer to be properly placed, but it will not work.
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<P>
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These components are:
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<UL>
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<LI>the transceiver chip,
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<LI>the microcontroller (<B>atusb</B> only),
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<LI>the balun, and
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<LI>the LED (<B>atusb</B> only)
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</UL>
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<!-- ---------------------------------------------------------------------- -->
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<SUBSECTION title="atben components">
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<B>atben</B> only has the transceiver and the balun. The markings on
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both face towards the crystal and the antenna:
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<P>
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<A href="atben-compor.jpg"><IMG src="atben-compor.png"></A>
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<P>
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The corner next to pin 1 of the transceiver is marked with a small dot.
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The picture above also shows the orientation of the text printed on
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the package. The balun is marked with a square between pins 1 and 6.
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<!-- ---------------------------------------------------------------------- -->
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<SUBSECTION title="atusb components">
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In <B>atusb</B>, the marking on the transceiver is on the corner between
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the crystal and the antenna. The marking on the balun faces towards the
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antenna. The microcontroller's "top" side faces towards the USB connector.
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<P>
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<A href="atusb-compor.jpg"><IMG src="atusb-compor.png"></A>
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<P>
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It is difficult to determine the LED's orientation by visual inspection.
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An efficient approach is to touch the LED's terminals with the probes
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of a multimeter set to measure resistance. The small current used for
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the measurement will light the LED.
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<!-- ====================================================================== -->
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<SECTION ref="voltages" title="Supply voltages">
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The transceiver has three voltage domains:
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<UL>
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<LI>The supply and I/O voltage, which is nominally 3.3 V in
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<B>atben</B> and <B>atusb</B>,
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<LI>the digital (core) supply, which is nominally 1.8 V, and
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<LI>the analog (RF) supply, which is nominally 1.8 V.
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</UL>
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<BR>
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On <B>atusb</B>, there is also the USB voltage domain at nominally 5.0 V.
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<P>
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Voltages should be tested in the following order: USB, then I/O, then
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digital, and finally analog. The table below gives the permissible
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ranges. Any voltages outside these ranges indicate a problem.
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<P>
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<TABLE frame="border" cellpadding="2">
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<TR><TH>Domain<TH>Nominal<TH>Minimum<TH>Maximum
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<TR><TD>USB<TD>5.0 V<TD>4.5 V<TD>5.25 V
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<TR><TD>I/O<TD>3.3 V<TD>3.0 V<TD>3.6 V
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<TR><TD>Digital<TD>1.8 V<TD>1.7 V<TD>1.9 V
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<TR><TD>Analog<TD>1.8 V<TD>1.7 V<TD>1.9 V
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</TABLE>
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<!-- ---------------------------------------------------------------------- -->
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<SUBSECTION title="Measurement setup">
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The measurements should be performed with a digital multimeter.
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The transceiver's analog and digital supplies (1.8 V) are only
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activated when sending or receiving.
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To enable all voltage domains, put the transceiver in receive mode:
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<PRE>
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atrf-txrx
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</PRE>
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or
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<PRE>
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atrf-txrx -d net:ben
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</PRE>
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Exit with Ctrl-C.
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<P>
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To produce periodic transmissions in addition to enabling all voltage
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domains, use
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<PRE>
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atrf-txrx -p 3 -E 0
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</PRE>
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or
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<PRE>
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atrf-txrx -d net:ben -p 3 -E 0
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</PRE>
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Again, exit with Ctrl-C. Note that the transmissions may disturb nearby
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equipment operating in the 2.4 GHz band, such as 802.11 networks. This
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can be prevented by shorting the antenna to ground.
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<P>
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In case the board does not accept commands, only the USB and I/O voltage
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can be checked. If they are correct, proceed with checking the clock.
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<!-- ---------------------------------------------------------------------- -->
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<SUBSECTION title="Measurement points on atben">
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The supply voltages on <B>atben</B> can be measured at the terminals of
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components as shown in this table:
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<P>
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<TABLE frame="border" cellpadding="2">
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<TR><TH>Domain<TH>Voltage<TH>Component
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<TR><TD>I/O<TD>3.3 V<TD>C3, C6
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<TR><TD>Digital<TD>1.8 V<TD>C5
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<TR><TD>Analog<TD>1.8 V<TD>C4
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</TABLE>
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<P>
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Ground can be accessed at the cover of the crystal.
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Note that the fiducials, while looking like test points,
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are not connected to anything.
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<P>
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This image shows the location of the measurement points:
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<P>
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<A href="atben-B.png"><IMG src="atben-B-small.png"></A>
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<!-- ---------------------------------------------------------------------- -->
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<SUBSECTION title="Measurement points on atusb">
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The supply voltages on <B>atusb</B> can be measured at the terminals of
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components as shown in this table:
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<P>
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<TABLE frame="border" cellpadding="2">
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<TR><TH>Domain<TH>Voltage<TH>Component
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<TR><TD>USB<TD>5.0 V<TD>C1
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<TR><TD>I/O<TD>3.3 V<TD>C2, C10, C13
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<TR><TD>Digital<TD>1.8 V<TD>C12
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<TR><TD>Analog<TD>1.8 V<TD>C11
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</TABLE>
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<P>
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Ground can be accessed at the cover of the crystal, at the shield of the
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USB connector, or at the test point P11.
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Note that the fiducials are not connected to anything.
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<P>
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This image shows the location of the measurement points:
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<P>
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<A href="atusb-B.png"><IMG src="atusb-B-small.png"></A>
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<!-- ====================================================================== -->
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<SECTION ref="clock" title="Clock frequency">
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The precision 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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This can pinpoint specific problems and help to select further analysis steps.
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<!-- ---------------------------------------------------------------------- -->
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<SUBSECTION title="Oscillator tolerances">
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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 for an initial assessment, and perform more precise measurements
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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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IEEE 802.15.4 requires the transceiver frequency to be accurate
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within +/− 40 ppm.
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<!-- ---------------------------------------------------------------------- -->
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<SUBSECTION title="Measuring the clock on atben">
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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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The clock signal (CLKM) is available on the test pad shown on the image
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on the left, and it should look roughly as shown in the screen shot on
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the right:
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<P>
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<A href="atben-A.png"><IMG src="atben-A-small.png" align="left"></A>
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<IMG src="scope-atben-clkm.png">
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<P>
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<TABLE frame="border" cellpadding="2">
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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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atrf-xtal
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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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<SUBSECTION title="Measuring the clock on atusb">
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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 microcontroller 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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The clock signal is available at the terminals of several components,
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either as the direct output from the transceiver (CLKM) or after passing
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a low-pass filter (CLK):
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<P>
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<A href="atusb-A.png"><IMG src="atusb-A-small.png"></A>
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<IMG src="scope-atusb-clkm.png">
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<IMG src="scope-atusb-clk.png">
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<P>
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The left screen shot shows the clock (CLKM) before the low-pass filter
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while the right screen shows the clock (CLK) after the the low-pass
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filter.
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<P>
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<TABLE frame="border" cellpadding="2">
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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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atrf-xtal (@@@)
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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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Note that, if testing a board into which no boot loader has been flashed
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yet, the clock frequency should be 1 MHz. If an unsuccessful attempt has
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been made to flash the boot loader, the frequency may be 1 MHz or 8 MHz,
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depending on how much code was successfully flashed.
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<!-- ---------------------------------------------------------------------- -->
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<SUBSECTION title="Precision measurements (atben)">
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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 atrf-xtal. atrf-xtal
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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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<PRE>
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ben# 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 on the same Ben at a comparable temperature.
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<P>
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<TABLE frame="border" cellpadding="2">
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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>Deviation can be compensated by adjusting trim value
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</TABLE>
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<!-- ---------------------------------------------------------------------- -->
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<SUBSECTION title="Precision measurements (atusb)">
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The clock frequency of <B>atusb</B> can be measured with an accuracy
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of about +/− 1 ppm relative to the PC's NTP-disciplined clock.
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<PRE>
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pc# atrf-xtal 10000
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</PRE>
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This measurement runs for 10−60 seconds, depending on system load.
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Note that, in order to reach such a high precision, it is imperative
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that the PC clock be disciplined by NTP.
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<P>
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<TABLE frame="border" cellpadding="2">
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<TR><TH align="left">Difference<TH align="left">Action
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<TR><TD>< +/− 30 ppm<TD>Correct operation
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<TR><TD>Other<TD>See the <B>atben</B> section above
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</TABLE>
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<END author="Werner Almesberger" date="<GEN_DATE>">
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</BODY>
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</HTML>
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