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PCI-E chips

Discussion in 'Electronic Design' started by David Lesher, Nov 28, 2009.

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  1. David Lesher

    David Lesher Guest

    I've been reading the basics of the PCI-E interface. Unlike PCI-nonE,
    which is parallel; PCI-E is serial, with slots/cards having 2^n, for n=0-5,
    serial "lanes" connecting them.

    A card can have less, equal or more lanes than the socket, except for
    physical space limits. [The socket may be 4 lanes wide; an 8 lane card
    won't fit. But it could be an 8-lane socket with only 4 implimented.]

    The motherboard/card automagically negotiate how many lanes work
    with a card. Further, newer lanes runs at higher bitrates than
    older ones, but can fall back for compatability.

    Besides flexibility, the serial scheme negates the issue of clock
    skew; it can handle one lane's data arriving later than others,

    Then there's some kind of crosspoint switch that gets the data where you
    want it. I'm told that SATA data gets similar treatment. I don't know
    if USB and FireWire also do.

    My curiousity is that switch. With a say 6 slot motherboard, 3 slots
    being 32 wide, and one each 16, 8 and 2.... add in the 4 SATA channels
    and ???, and you have a LOT of channels into that crosspoint switch. And
    at the coming 1GB/s rate per lane; that's a lots of bits.

    How big are those crosspoint chips, anyhow?
  2. Jamie

    Jamie Guest

    Sounds like you may be talking about the original micro channel idea
    from the days of the Micro channel technology from IBM..
    Or maybe it's not?
  3. Each lane cannot be arbitrarily connected to any other lane on the bus. Also
    there are only a fixed number of lanes(I think 64 for 2.0). This is why two
    video cards, even though both are x16, will only run at x8 each. One gets
    half the lanes and the other gets the other half. Ultimately the pci-e data
    is packetized and put on the internal bus just as any other data is.

    The lanes are simply there to replace a normal parallel bus since it
    modular. One can remove or add lanes without issue. Interally the device
    sending data across it just sees less latency when more lanes are in use.

    In its early implementations, motherboards capable of SLI required a special
    card ("paddle card") which came with the motherboard. This card would fit
    into a socket usually located between both of the PCI-Express x16 slots.
    Depending on which way the card was inserted, the motherboard would either
    channel all 16 lanes into the primary PCI-Express x16 slot, or split lanes
    equally to both PCI-Express x16 slots. This was necessary as no motherboard
    at that time had enough PCI-Express lanes for both to have 16 lanes each.
    With the increase in available PCI-Express lanes, most modern SLI-capable
    motherboards allow each video card to use all 16 lanes in both PCI-Express
    x16 slots.

    The SLI bridge is used to reduce bandwidth constraints and send data between
    both graphics cards directly. It is possible to run SLI without using the
    bridge connector on a pair of low-end to mid-range graphics cards (e.g.
    7100GS or 6600GT) with Nvidia's Forceware drivers 80.XX or later. Since
    these graphics cards do not use as much bandwidth, data can be relayed
    through just the chipsets on the motherboard. However, if no SLI bridge is
    used on two high-end graphics cards, the performance suffers severely as the
    chipset does not have enough bandwidth."
  4. David Lesher

    David Lesher Guest

    Hmm, I've not seen that limit before.

    One aspect I can really cheer over is the two-way backward compatabilty.

    Not only can a PCIE2 motherboard slot accept PCIE1 cards; but a PCIE1
    motherboard can accept PCIE2 cards. Ditto for PCIE3.... the autodiscovery
    negotiates all.

    Someone, somewhere, REALLY had their head on straight on that one.

    No more of this "It's PCI, but you have the wrong voltage on that socket"
    and so forth.
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