PCB Design for Crystal & Eagle

Discussion in 'The Projects Forum' started by Corentin, Oct 2, 2012.

  1. Corentin

    Thread Starter New Member

    Oct 2, 2012
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    0
    Good morning,

    I have experienced some trouble with crystal oscillators lately so I wanted to design my new PCB respecting 'the rules'.

    My board has a microcontroller MSP430 in QFP package with an external 32,768Hz crystal oscillator. According to the following TI's document, at page 6 and 7, "PCB Design consideration" chapter:
    http://www.ti.com/lit/an/slaa322b/slaa322b.pdf

    1./ The traces to the crystal must be as short as possible.
    2./ The other traces must be kept away from the crystal, even on the opposite layer.
    3./ XIN and XOUT must be kept away from each other.
    4./ A Guard Ring should be built around the crystal and connected the AGND (analog ground pin of the microcontroller). A ground plane should be filled on the opposite layer and also connected to AGND. The connection to AGND should be short.
    5./ "Never connect the ground guard ring to any other ground signal on the board."

    Please find attached my PCB designed on Eagle. I probably haven't respected all the good use to use Eagle, but I keep learning with this software. Please see the crystal next to the text CRYSTAL, on the bottom layer.

    My questions are:
    1./ Is the connection to AGND short enough ?
    2./ AGND must be connected to GND at some point on the PCB for some other reasons (Analog-to-Digital Conversion). Will it break the rule "Never connect the ground guard ring to any other ground signal on the board." ?
    3./ Are the planes (guard rings) on top and bottom layers properly designed ?
    4./ Is the crystal too close to the microcontroller ?
    5./ Any other comment regarding the crystal ? I have only one shot for this PCB.

    Thank you very much for your help
    Corentin
     
  2. nerdegutta

    Moderator

    Dec 15, 2009
    2,519
    785
    Hi.

    Not everybody has Eagle installed on their computers. I think you should export an image of the board as *.png and upload that.

    File -> Export -> Image

    (I think that's the way...)
     
  3. Corentin

    Thread Starter New Member

    Oct 2, 2012
    5
    0
    Hi,

    see attached the PNG file.

    I tried to highlight the AGND route but it didn't work. So, as you can see, from the Guard Ring plane a route goes to the AGND pin of the microcontroller. But the route continues and also goes to a jumper to join with GND at some point on the board.

    I added a via so that the Guard Ring on bottom layer (layer where the crystal is soldered) is also connected to AGND.

    Edit:
    to give you more information. The board size is 62x27 mm², the spacing between routes is 0.20mm.

    Corentin
     
  4. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
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    Will this board be made in board house? If you have to hand solder every via. It could be a problem to solder in a chip on top of those vias.
     
  5. Corentin

    Thread Starter New Member

    Oct 2, 2012
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    0
    The PCB will be manufactured in a board house. I will hand solder the board, but I have quite a good equipment and I am able to do it. At least, I was with the previous boards. But you are right, I need to pay more attention to the position of some vias. Thanks.
     
  6. t06afre

    AAC Fanatic!

    May 11, 2009
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    It looks OK to me.
     
  7. nerdegutta

    Moderator

    Dec 15, 2009
    2,519
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    Some questions/comments:


    1. Why is this VIA different from the others?
    2. Why the extra routing?
    3. Why this?
    4. Strange 90° bend.
    5. Strange routing.
    Maybe I don't see the whole picture...?
     
  8. Corentin

    Thread Starter New Member

    Oct 2, 2012
    5
    0
    Thank you for your answers and comments.

    1. I made the via bigger hopping to improve the signal (kind of reducing the route length). But it musn't have any effect. I'll set the via back to its normal size.

    2, 3, 4, 5
    Thank you very much for your comments. I used Eagle Auto-router. And as I said, I don't have the full software under control yet. The Auto-router made the 90% of the routes, I had to finish manually the 10%, and it wasn't the easiest one. I'll retouch the routes.

    Otherwise, does the crystal design look okay ?

    Again, thank you for your help.
    Corentin
     
  9. t06afre

    AAC Fanatic!

    May 11, 2009
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    You have to move quite high up in frequencies before tracks at right angles causes trouble. Tracks at right angles may cause "shadows" then etching PCB. And hence cause imperfect etching
    So a tip. Then you think your board is done. Print out the board in size 1:1. Then place components on the printout to see if they fit. Components worth checking are "big" components like electrolyte caps, inductors, etc. It could be that you find out that your crystal is placed to close the CPU. So if wont fit. As an example
     
  10. Corentin

    Thread Starter New Member

    Oct 2, 2012
    5
    0
    Thank you for the tip.
    I'll do that and check if everything's okay.

    Corentin
     
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