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What's the purpose of R20 in this schematic

  1. Feb 18, 2009 #1
    I'm studying a portion of a schematic from an evaluation board. What's the purpose of R20 in the attached schematic?
     

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  2. jcsd
  3. Feb 18, 2009 #2

    mgb_phys

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    Isn't it just a connection between the two clocks?
    It's common to draw a link as a zero Ohm resistor in some packages, it lets the layout software actualy put a real part there so you can use it as a test point, or remove it to test the two clocks separately. On surface mount it's the normal way to do a link.
     
  4. Feb 18, 2009 #3
    ja, it's a jumper
     
  5. Feb 18, 2009 #4
    I'm not so sure that it's just a jumper...

    Here's a snippet from a similar eval board. In this schematic the resistor is R35. Any ideas?

    The full schematic for this snippet can be viewed is at http://www.olimex.com/dev/images/ARM/ATMEL/SAM9-L9260-REV-B-sch.gif" [Broken]
     

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    Last edited by a moderator: May 4, 2017
  6. Feb 19, 2009 #5

    berkeman

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    Hard to say. 100 Ohms is different from the zero Ohm jumper in your first post. 100 Ohms is more like a termination, but zero Ohms is almost always a jumper for stuff options.
     
  7. Feb 19, 2009 #6
    Thanks for all the input.

    I still don't know what the 100 Ohm resistor is for but my theory for the zero Ohm resistor is that it gives the user the option of accessing the SD card via SPI or SDIO. Does that make sense?
     
  8. Feb 21, 2009 #7
    R20 allows the user to use either common or separate offboard clocks.

    R35 allows offboard selection of one of two clocks. If you happen to erroniously connect both, the 100 ohms will protect the offboard clock drivers.
     
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