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RE: PCI 5V signal
(Well, this will be try #3, as the other two didn't ever make it to znyx.com
and it's been longer than 3 days.)
> I am new to PCI Bus. i am just wodering why do we need so many 5V finger
> pins on the edge connectors? There are a total of 13 5V pins, especially
> at the end corner of the edge connector, 4 5V pins seat next to each
> other. Do i have to connect them all to the 5V power plane?
>
> Yes, you do need to connect to all of them.
>
> All of them do not need to connect to your +5V plane (if your "plane"
> doesn't cover the whole board, and if your +5V current is well under the
> maximum), but all of them DO need to have finger pins and be decoupled
> near the connector.
>
> They do two things. They bring +5V power onto your card, and if you have
> too few pins and draw too much current, you could literally burn out the
> pins, if not degrade the +5V power quality. Also, all the power and
> ground pins are part of the signal "return path" through the connector,
> for signal integrity. They, like the ground pins, can be thought of as
> "shield" pins; but it's more than that.
>
> Each signal pin needs a return path, as nearby as possible. If there
> isn't one, you get lots of crosstalk, poor waveforms, and timing that
> doesn't work. Ground works best as a return path, but for this purpose,
> power and ground pins can be treated equivalently when the power pins are
> connected and well bypassed on both sides of the connector. By not
> connecting any of those pins, whether ground or power, you compromise the
> connector's signal-to-return ratio and the integrity of nearby signals.
>
>
> I was adviced to put some low speed tantalum capacitor(minimum 10 uF),
> located no more than 300 mils from the PCB's power entry points(edge
> fingers). This is used for power decoupling. Do i have to connect the
> tantalum cap to each 5V pins. There are 13 of them in total, so the caps
> would take up a lot of space.
>
> Low speed capacitors can be placed anywhere on your board. There is
> certainly no need to have them close to any pin, and no need to have one
> per pin. One (of appropriate size) per board may be OK.
>
> What you should have, are high speed caps as close as possible to each +5V
> pin. Caps can be shared between pins that are very close together, such
> as the four at one end.
>
>
> Also i need some high speed caps(0.01uF) to decouple any unused 3.3V pins
> on the edge connectorto the ground plane. There are 12 3.3v pins there!
> So i would need to locate more than 20 capacitors near the edge
> connector!! Even if i can bus a few power pins onto one cap, it still take
> up a lot of space. I am runnning out of space now.
>
> You do not need literally one 0.01uF cap at each pin. A single cap (0.047
> or 0.1uF for example) can be shared by a few power pins that are close.
> See section 4.4.2.1 in the PCI Spec. "There is no limit to the number of
> pins that can share the same capacitor" provided that they are within 250
> mils and there is at least 0.01uF of capacitance per power pin.
>
>
> so i am wondering how do you guys usually tackle this problem?? How many
> capacitors(both low and high speed) do you guys used. I have seen one
> example PCI card with only a few caps to decouple 3 to 4 3.3V pins, there
> are still a few unused 3.3V there without decouple caps. Also they didn't
> have any caps for the 5 V signal pins.
>
> If a board has missing +5V or +3.3V (or +Vio or GND) pins, it does not
> comply with the PCI specs and should not be sold. The same is true if its
> decoupling on those pins is inadequate. They run the risk of being
> incompatible with some PC systems, causing them to freeze or crash.
>
> It is true that a board with solid Vcc and GND planes has some high-speed
> intrinsic capacitance between those planes, and some might argue that this
> would eliminate the need for actual high speed caps. However, the
> intrinsic capacitance is generally too small and some capacitors need to
> be added, especially in the vicinity of points where switching current
> needs to enter or exit the planes (at connectors and ICs).
>
>
> Just for your information, i am designing a 5V 32bits PCI add-in card.
>
> The current recommendation, is that everyone should be designing only
> "Universal" PCI expansion cards which can be plugged into both 5V and 3.3V
> systems. It is thought that at some point in the near future, 3.3V
> systems may begin to proliferate, and 5V cards are worthless for a 3.3V
> system.
>
> Regards,
> Andy
>
>