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Do all of the VDDs and VSSs need to be connected?? — Parallax Forums

Do all of the VDDs and VSSs need to be connected??

laser-vectorlaser-vector Posts: 118
edited 2010-11-05 18:16 in Propeller 1
Hey, im making up a PCB using the LQFP propeller and i was wondering if it was essential to connect all of the VDDs and VSSs???

Thanks

Comments

  • SapiehaSapieha Posts: 2,964
    edited 2010-10-06 11:49
    Hi

    YES.
    And all need decoupling's Capacitor
    laser-vector wrote: »
    Hey, im making up a PCB using the LQFP propeller and i was wondering if it was essential to connect all of the VDDs and VSSs???

    Thanks
  • laser-vectorlaser-vector Posts: 118
    edited 2010-10-06 11:56
    Sapieha wrote: »
    Hi

    YES.
    And all need decoupling's Capacitor

    i remember running a dip40 for a very simple project using only one VDD and one VSS, so i wasnt sure... this project is a little tight on the front layer, i guess ill just via all of the VDDs and VSSs to the back layer and put the SMD capacitors right under the other side of the propeller.

    Thanks!
  • ElectricAyeElectricAye Posts: 4,561
    edited 2010-10-06 12:01
    About bypass caps:

    http://forums.parallax.com/showthread.php?t=122212&highlight=bypass
  • laser-vectorlaser-vector Posts: 118
    edited 2010-10-06 12:02
    as for the capacitors, it looks like it calls for no-polarized (ceramic) caps???

    will these suffice http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=490-1575-1-ND

    some people say to use a larger tantalum capacitor but im not sure id need it for what im doing
  • RobotWorkshopRobotWorkshop Posts: 2,307
    edited 2010-10-06 12:06
    laser-vector wrote: »
    as for the capacitors, it looks like it calls for no-polarized (ceramic) caps???

    will these suffice http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=490-1575-1-ND

    Yes, those should be fine.

    If you use the DIP version for more projects you should really use all the VDD and VSS lines as well.

    Robert
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-10-06 12:17
    Laser-vector,

    Don't forget: you need bypass caps and a filter cap. Ideally, a ceramic bypass cap is placed near each Vdd/Vss pin pair for each IC on the board -- not just the Prop. The filter cap gets located near the voltage regulator. You will have to consult the regulator's datasheet to determine what kind and what value filter cap to use. Most regulators also require an input cap.

    -Phil
  • RobotWorkshopRobotWorkshop Posts: 2,307
    edited 2010-10-06 12:29
    If you end up using a Tantalum cap at the regulator there are a couple things to consider. I've usually taken a conservative approach and use them at 1/2 of their rated working voltage. One datasheet that mentions the "added reliability" of derating the caps is here:

    http://www.kemet.com/kemet/web/homepage/kechome.nsf/weben/3AB934DEB66DBD91852571E90068CBE4/$file/F3113_Mil_TA_SM.pdf

    The other concern is with the marking. The striped end on those is usually the positive side (rarely mentioned in the datasheets) instead of just about all other components which use the striped end as the negative. That got me the first time I used these surface mount tantalum caps......

    Robert
  • Nick McClickNick McClick Posts: 1,003
    edited 2010-10-06 14:46
    @Robert - I figured out the striped-end issue the hard way one time. Surprised to see it start smoking so quickly...

    The prop will probably work if you don't connect all the VDD / VSS pins, but it is poor practice and will cause reliability issues. In a pinch - maybe. But if you're laying out a board, you should definitely connect all the VDD / VSS pins and use bypass caps (and filtering caps for the VReg, too).
  • Cluso99Cluso99 Posts: 18,069
    edited 2010-10-06 15:47
    You MUST connect ALL Vss and Vdd pins on the prop. Otherwise you might expect to blow the internal PLL at some time in the future!
  • laser-vectorlaser-vector Posts: 118
    edited 2010-10-06 16:31
    For those of you who know ExpressPCB this is a simple two layer PCB using the QFP propeller on the front layer and some SMD decoupling capacitors on the back layer.

    hows it look?:

    CpaMask.jpg

    [EDIT]

    on the top left of the image (the two traces for power and ground) the left one is 3.3vdc and the right one is ground..
  • hover1hover1 Posts: 1,929
    edited 2010-10-06 18:31
    First quick look shows the bottom Cap off of VDD is just in series with VDD, and not connecting to VSS. Were you wanting to go straight up to VSS?

    Edit: Plus make all smaller VDD, VSS traces fatter, to match to others.

    Jim
  • LeonLeon Posts: 7,620
    edited 2010-10-06 22:16
    Those angled tracks look untidy, they should all be at 45 degrees.

    You can avoid those angled connections to the pads in the lower left corner by staggering the vias.

    You have an acute angle on one of the tracks where it joins the fat supply track, which should be corrected.

    Mitre those right angle bends.
  • laser-vectorlaser-vector Posts: 118
    edited 2010-10-07 08:14
    @ Hover,

    thanks for the sanity check, yea that cap at the bottom was laid out wrong.. should be fixed now. plus i thickened up the traces ;-)

    @ Leon,

    i agree, the board was looking a little untidy.. i spent a little time last night before i went to bed and cleaned it up some. i like that staggering idea too!


    thanks everyone! and if you see anything else that is not up to par please do tell

    QFPPROP2.jpg

    Silkscreen Disabled:
    systemcontrol.jpg

    Silkscreen Enabled:
    systemcontrolsilk.jpg
  • LeonLeon Posts: 7,620
    edited 2010-10-07 08:25
    You still have several connections between tracks with acute angles; they should meet at right angles. It looks neater and avoids acid traps when etching.
  • laser-vectorlaser-vector Posts: 118
    edited 2010-11-05 15:33
    just wanted to say thanks to everyone for all the help and give a little feedback!

    well here it is: system control rev 1.0

    PCBMTRCTR.jpg
    DSC04484.jpg
    DSC04483.jpg

    this is my first real propeller project and wont be the last!! :D
  • hover1hover1 Posts: 1,929
    edited 2010-11-05 17:36
    Looks good!
    I love when green squares/rectangles come together. :)

    Jim
  • laser-vectorlaser-vector Posts: 118
    edited 2010-11-05 18:16
    hey thanks hover!

    this being my first PCB it was very exciting to see it work first try!
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