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Capacitors on Power Pins
- To: Mailing List Recipients <email@example.com>
- Subject: Capacitors on Power Pins
- From: "William Benner" <firstname.lastname@example.org>
- Date: Tue, 8 Sep 1998 01:45:25 -0400
- Resent-Date: Tue, 8 Sep 1998 16:27:23 -0700
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I have a few questions about the use of decoupling and bypass capacitors on
the power pins of a PCI plug in board.
I thought that I had read somewhere that each PCI plug in board must have
capacitors on the power pins, even if these pins are not used by that
particular board. But while examining other people's PCI boards, I don't see
these. Am I mistaken or is this some minor violation of the spec?
Another question, while designing my "Universal" 3V/5V board, I believe that
the so called VIO pins will go unused by my design because I am using a 3.3V
PCI chip that is 5V tolerant. I believe that the Gods that put together PCI
did not anticipate such "5V tolerant but 3.3V powered" devices when they
created the spec. Can I simply leave these VIO pins unconnected, or must I
connect them and use a capacitor?
Last question (for now), how exactly does this whole 3.3V powered devices
which connect to the PCI bus "work" in a "reflected-wave switching"
environment like PCI? According to PCI System Architecture book, "A
carefully selected, relatively weak output driver is used to drive the
signal line halfway to the desired logic state..." How does a 3.3V chip know
what "halfway" is when it does not know if it is a 3.3V or 5V signaling
environment? It must work because so far I have seen two PCI manufacturers
jump on this method of making a universal chip.