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Prop at 100mhz, what caps across vss and vdd? — Parallax Forums

Prop at 100mhz, what caps across vss and vdd?

HollyMinkowskiHollyMinkowski Posts: 1,398
edited 2010-04-09 04:08 in Propeller 1
I need to put some caps across the pwr leads of the 40pin dip prop.

I have access to some really cheap 10uf 6.3v tantalums.
Are these adequate? or is the 6.3v rating not high enough..

Should I use one of these also across pins 9 and 12 or am I ok
with just the one 10uf across 29 and 32...near the xtal?

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-04-08 22:43
    Your tantalums should be fine as filter caps (depending on you 3.3V regulator's specs), and you should have one across Vdd-to-Vss. (Yes, 6.3V is high enough.) But you still need some decoupling caps on each Vdd-Vss pin pair. 0.1uF ceramics will work fine for this.

    -Phil
  • Mike GreenMike Green Posts: 23,101
    edited 2010-04-08 22:47
    It's helpful to have something like the 10uF 6.3V tantalum capacitor across the power leads, but it doesn't substitute for several 0.1uF ceramic bypass capacitors between each Vdd/Vss pair. The 10uF capacitor has too much inductance to adequately provide the nanoseconds long power surges of the Propeller. You need at least one 0.1uF ceramic bypass capacitor across each Vdd/Vss pair. You need more than one because the power has to make it across the chip otherwise through the metallization rather than directly from the external pins. Without the multiple sources of power, the resistance of the metallization comes into play and you'll get surges across the chip which can be destructive.
  • CannibalRoboticsCannibalRobotics Posts: 535
    edited 2010-04-08 22:48
    This got me started on series and parallel cap math. Anybody know/remember how that effects the voltage capacity? ie. Parallel cap values are added, is the voltage added too?

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  • James NewmanJames Newman Posts: 133
    edited 2010-04-08 23:07
    No, that is the max voltage that cap can handle. You want to go 2x what you might actually see across it.

    More info on caps here: http://www.allaboutcircuits.com/vol_1/chpt_13/4.html
  • HollyMinkowskiHollyMinkowski Posts: 1,398
    edited 2010-04-09 00:19
    I have just been looking at schematics of various propeller boards.
    Many seem to have no caps connected across the vdd-vss pairs. ?

    So, does the prop usually work just fine even without these?

    A new qfp prop board was just put up on gadget gangster for instance.

    It has a 4.7uf 10v tantalum across vss-vdd pins 17 and 18 but no ceramic.

    Vss-vdd at pins 27 and 30 seem to have no cap at all, and this is
    the pair nearest the xtal pins.

    The schematic does not show caps across any vss-vdd pair...it even
    left off the 4.7uf that can be seen in the photo across 17,18.

    Is it that only the dip prop needs these caps while the sm versions do not?

    pZedma0P.jpeg

    Post Edited (HollyMinkowski) : 4/9/2010 12:34:25 AM GMT
  • ElectricAyeElectricAye Posts: 4,561
    edited 2010-04-09 00:37
    Mike Green said...
    ...You need more than one because the power has to make it across the chip otherwise through the metallization rather than directly from the external pins. Without the multiple sources of power, the resistance of the metallization comes into play and you'll get surges across the chip which can be destructive.

    Is this "destruction" a stability problem alone or does it actually reduce the life expectancy of the chip? A few of my older designs did not have bypass caps on all Vdd's so I'm wondering if I should go back and try to solder a cap from the pin to the grounding plane.

    sad.gif
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-04-09 00:40
    Holly,

    Not everyone follows the best engineering practices. You've gotten good advice in this thread, though.

    -Phil
  • HollyMinkowskiHollyMinkowski Posts: 1,398
    edited 2010-04-09 00:40
    @David Jensen

    That part seems to be across BOEn and VDD not across VSS-VDD
    Whatever it is it was left off the schematic.

    R8CcVRUp.jpeg
  • HollyMinkowskiHollyMinkowski Posts: 1,398
    edited 2010-04-09 00:50
    Even the prop demo board from Parallax only has one 1uf cap across
    qfp pins 5 and 8...the other vss-vdd pairs connect directly to +v and gnd.
    z3Wqysxv.jpeg
  • HollyMinkowskiHollyMinkowski Posts: 1,398
    edited 2010-04-09 03:52
    Since the demo board uses a 1uf cap across at least one set of pwr pins I
    ordered 100 cheap 1uf multilayer ceramics to try on my board...hopefully
    that will be enough. I will place one very close to each set of pwr pins on
    the 40 pin dip and see if it runs reliably at 100mhz. I will try it with and
    without additional 10uf tantalums.
    ZCdPnT83.jpeg
  • Cluso99Cluso99 Posts: 18,069
    edited 2010-04-09 04:04
    Holly, the problem is more than just if it works. There have been many circuits designed that worked in the R&D lab but were incorrectly designed in the first place and then failed miserably in the field.

    If you want to overclock, you really should also include the tantalum. It actually depends on what code you are running on the prop that determines the requirements and we cannot predict that so we should design for worst case.

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  • Nick McClickNick McClick Posts: 1,003
    edited 2010-04-09 04:05
    @holly - the caps used on the Propeller Platform SD are on the next page of the schematic.

    I don't know if the demoboard (propstick and protoboard have similar schematics) follow best practices, but I haven't had any problems with them or the Propeller Platform.

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  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2010-04-09 04:08
    Holly, occasionally you will see 0.01uf specified instead of even 0.1uf. Once again this is usually to do with the high-frequency response of the "decoupling" capacitors, they have a lower impedance at higher frequencies than their larger cousin. 1uf is probably fine but not as good as a 0.1uf for high-frequency decoupling but ten 0.1uf caps paralleled to equal 1uf would be better still, their parasitic series inductance or high-frequency ESR lowers as you parallel them improving high frequency response.

    The BOE line BTW is usually connected to ground so the pcb you saw would have the cap across VDD and GND. It is not necessary to place the big caps close to the Prop as they are mainly there to handle the "brown-outs" vs the "transients" that the fast X7R ceramics handle.

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