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PCI bus

Discussion in 'Electronic Design' started by [email protected], Aug 17, 2004.

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  1. Guest

    I have some custom PC boards that years ago, I redesigned from the ISA to PCI
    interface via custom PLX chips. They have always performed with no problems,
    but the customer PCs were a bit behind the curve, and all ran with sub
    gigahertz CPUs.

    When I try to use them in MBs with CPUs exceeding 1.2GHz, the PC won't boot
    with those same cards installed in the PCI slot. I suppose I also may be
    having an issue in that the newer boards are using AMD processors rather than
    Intel Pentiums.

    Before providing tons of what may be useless information, can anyone vouch for
    any information or theories that the PCI slot timing is changing as the CPU
    speeds increase? I am of the impression that a PCI slot is a PCI slot, and
    that it is standardized, but that is not what I am seeing, so I am just asking
    in a general sense if there is a known issue here before I get in over my

    Much obliged for an insight or ideas.


  2. PaulCsouls

    PaulCsouls Guest

    The PCI bus is spec'd to be 33MHZ or 66MHz for 66MHz PCI, but on some
    older motherboards the PCI clock is generated as a percentage of the
    front side bus. If you install a new CPU and change the jumpers
    settings for the FSB to increase the speed of the processor you will
    increase the speed of the PCI bus. My computer is over clocked to
    generate a 75MHz FSB and a 37MHz PCI bus for a 450MHz AMD K6-2. My PCI
    bus runs just fine at 37MHz. If I increase the bus to 83MHz to get
    500MHz out of my microprocessor, the 41MHz PCI bus will lock up my

  3. Iwo Mergler

    Iwo Mergler Guest

    Since the PCI timing is subject to a standard, there should be
    no change. Paul is right, of course, the PC may violate the
    standard, depending on clock settings.

    Even without actually violating the standard, modern mainboards
    may be less forgiving if your card 'stretches' the standard a
    little. I would re-check the layout. In a nutshell - min. 4-layer
    board, solid power planes, PCI traces less than 1 inch, clock
    trace 1.5 inch, only one cmos input/driver per signal.

    Another thing that comes to mind is power. If the card causes
    a large current surge at startup, it could push an already
    struggling power supply over the edge.

    Kind regards,

  4. Also, PCI may be anything *below* 33MHz. The old system may have had
    an under clocked PCI. Paul mentioned that his 75MHz FSB machine over
    clocked the PCI bus. The other alternative was to under clock the PCI
    bus by dividing the FSB by three (25MHz, in this case). Others used an
    asynchronous PCI bus at 32MHz and divided that by 4 to the 8MHz ISA bus
    (others over-clocked the ISA bus slightly).
    I believe the spec on the clock trace is 2.5" +/- .1". least
    that's what Shanley and Anderson say. I'd have to look for my copy of
    the spec. I've gotten away with violating this spec before (oops), but
    it's someplace to look.
    I'd think the newer systems would be more forgiving here, but it's a
    good thought too.
  5. Nico Coesel

    Nico Coesel Guest

    I don't think the type of CPU has much to do with it (altough I highly
    recommend using Intel CPU's in servers / industrial applications
    because they have more thermal headroom and therefore are more
    reliable). The non-Intel chip set may be the problem.

    On the other hand, the newer PCI buses require stricter timing.
    Although they claim these buses to be compatible with PCI 2.1, they
    aren't. I've had similar problems with the Xilinx PCI core. When I
    upgraded to the latest core, the problems where over.
  6. SioL

    SioL Guest

    Compare power ratings of Athlon 64 with the latest generation of P4 (Prescott)
    or the other P4 high end parts. You might be surprised.

  7. Nico Coesel

    Nico Coesel Guest

    Do the thermal math on the packages and you'll find the AMD packages
    allow the die to get hotter than Intel's packages while being within
    operational specification. In other words: AMD CPU's are like a tuned
    race engine. You can drive very fast with it, but it won't last very
    long. Besides, Intel has already shown they design towards reliability
    by adding over heating protection into the P3 (and P4) long before AMD
    thought of doing that.
  8. Pooh Bear

    Pooh Bear Guest

    How does that affect the operation of the PCI bus exactly ?

  9. SioL

    SioL Guest

    Are you talking about the old Athlon regular version without the metal
    heat spreader with the practically exposed die?

    Anyway, AMD now has that power down diode included. And I've
    never heard of a person with that kind of a failure on AMD or Intel.

    Any decent server board will detect a failed fan and shut down the pc anyway.

    It seems to me that today any processor becomes obsolete long before its
    life span is exceeded.

    Plus, Athlon64 has that buffer overrun protection supported in WinXp sp2,
    which Intel won't have for some time to come.

    Not to mention that Intel was forced to go AMD's route with the 64 bit.

    Right now I don't see any reason to buy a slower & more expensive part
    from Intel. Even the chipsets have matured.

  10. Guest

    Thanks to all who provided me some insight into dealing with the PCI bus
    problems. I appreciate the assists.

    I hope its merely a case of a BIOS setting for timing and not a need for
    rethinking the design. <shrug> Guess I will find out soon enough. Seems
    strange that it would cause the PC not to boot if it was just a timing thing,
    but I am hoping for the easier solution. :)

    Much obliged for your time.
  11. MM

    MM Guest

    Which PLX chip are you using? I have designed quite a few PCI cards and I am
    100% sure that the CPU speed has nothing to do with this (assuming no weird
    reclocking has been done). How does it not boot? Where does it stop? Do you
    get BSOD or what? I assume your card is a 5V card. Make sure that the 3.3V
    power pins are not shorted to anything. In the older PCs they frequently
    were not used, but since PCI 2.2 it is a requirement, so all the new
    machines supply it to the slots... Can you dump the PCI configuration space
    of your card on a PC where it works? That might help to diagnose the

  12. Guest


    I am going to visit the company next week and see what is going on. I will
    certainly check into what you have mentioned, and if you don't mind I will
    get back to you with the info on what happens at bootup, as I have not
    actually seen the problem yet.

    The PLX chip is the 9050, and thanks for the advice about the power supply
    lines. I will certainly check on that as well.
    I must confess that I am not sure what you are referring to here. Can you
    clarify it for me?

    Thanks for your time and assistance.

  13. MM

    MM Guest


    PCI spec requires every PCI device to implement certain registers, where
    information about the device is stored. When the system boots it scans the
    PCI bus and reads the configuration space of each device it found. Based on
    this information it assigns system resources to the devices, such as memory
    space, I/O space, interrupts and DMA channels. Sometimes the system might
    crash if it finds a device that requires too much resources, although it is
    supposed to come out of this situation more gracefully by disabling the
    offending card. On your card, do you have an EEPROM to configure the PLX
    chip or are you relying on its default configuration? If the latter then
    there is not much point in dumping the configuration data, since PLX default
    is fine, but if the former then you should look at this data closely. There
    are a few utilities that you can find online that allow to scan PCI devices.
    Some of them can dump the whole space into a file... I don't have any links
    but try googling... Also try going to website, they used to
    have a collection of links to this kind of stuff. PLX used to give away a
    utility. My favorite has been PciView from BlueWater Systems. It seems to be
    hard to find these days since this company doesn't exist anymore, but I can
    send it to you on Monday if you want. The version I have doesn't do file
    dump but will let you see the full configuration space.

  14. PaulCsouls

    PaulCsouls Guest

    configuration mechanism #1
    BIOS major version 2
    BIOS minor version 16
    Last PCI bus in system 1

    PLX9054 device found
    Class: 6 SubClass:80 Program IF: 0
    Bridge Devices: Other
    Device ID:9054 Vendor ID:10b5
    Sub System ID:9054 Sub Vendor ID:10b5
    Device Number: f Function Number: 0
    Bus Number: 0 Index: 1
    Status:290 Command:117
    PCI IO
    PCI memory
    Bus master
    Memory write and Invalidate
    SERR enabled
    New Capabilities List
    Fast Back to Back
    Device Select Timing: Medium
    BIST: 0 HDR: 0 Latency:40 Cache Line Size: 8
    Base Address 0: febeef00
    Memory Lower 4GB
    Size 256 Address febeef00
    Base Address 1: e401
    I/O Size 1024 Address e400
    Base Address 2: febed000
    Memory Lower 4GB
    Size 4096 Address febed000
    Base Address 3: febe8000
    Memory Lower 4GB
    Size 32768 Address febe8000
    Base Address 4: 0
    Base Address 5: 0
    CardBus CIS Pointer: 0
    Expansion ROM Base Address: 0
    Capabilities Pointer: 40
    Capabilities Register 0: 14801
    PCI Power Management Interface
    Capabilities Register 1: 804c06
    CompactPCI Hot Swap
    Capabilities Register 2: 3
    Vital Product Data
    Max_Lat: 0 Min_Gnt: 0
    Interrupt Pin: 1 Interrupt Line: 9

    Size 256 Address febeef00 Selector 0 cf
    Size 1024 Address e400 Selector 1 d7
    Size 4096 Address febed000 Selector 2 df
    Size 32768 Address febe8000 Selector 3 e7
    DMA 0 Local Bus Width 16
    DMA 0 Wait States 0
    READYN enabled
    Local Burst enabled
    Local Address Bus Constant
    Demand Mode enabled
    Slow Terminate Mode
    DMA 0 PCI Address 1f80000
    DMA 0 Local Address 0
    DMA 0 Transfer Size ff
    DMA 0 Descriptor Pointer b
    Mode DMA Arbitration 280000
    DMA Threshold 1111
    DMA DAC 0
    Interrupt control status f010180

    DMA Config 80: 1941
    DMA Config 84: 1f80000
    DMA Config 88: 0
    DMA Config 8c: ff
    DMA Config 90: b
    DMA Config 94: 43
    DMA Config 98: 0
    DMA Config 9c: 0
    DMA Config a0: 0
    DMA Config a4: 0
    DMA Config a8: 1010
    DMA Config ac: 280000
    DMA Config b0: 1111
    DMA Config b4: 0
    DMA Config b8: 0

    I have some DJGPP code that does this but it in only works in DOS or
    Win95-98. You can probably find a more up-to-date utility if you hunt
    around. I think the PLX debug utility will do it.

  15. Dana Raymond

    Dana Raymond Guest

    I have read this thread, and agree with much of the good advice (CPU deson't
    matter, cpu overclocking can race PCI bus depending on chipset, shorted vio
    and other power lines, etc.)

    My first question is: To what spec did you design the PCI card?
    From there I would see the differences.

    One thing you should check for is your card edge keying. Also, does the MB
    key for 5V and 3.3V, or only one? That keying is what prevents older PCI (5V
    only) cards from being plugged into newer MBs.

    I am a little concerned that the card isn't multilayer! You usually need a
    GND plane to maintain the proper impedance on the signalling lines (return
    path on GND layer).

    Keep in mind that the PCI spec only allows for so many 'loads'. Each
    connector counts as 1 load and another load for the card plugged into it.
    But, a lot of MBs have on-board PCI devices that count for only 1 load.

    Please, start with your card keying, the PCI keying on the MBs that work and
    those that don't, and confirm that its not likely a PCI clock frequency
    thing. You can check the PCI clock with a scope or counter - its not
    reflective. We can go from there.

    Oh yes, I would definately try modding a card and holding the PLX in reset.
    That will tell us if the problem is purly electrical or if there is a bus
    confic problem , or the PLX is programmed for FAST response and the MB's
    can't handle that, etc.

    Mark... If you like I can take a quick courtesy look at your schematics and
    layout if provided in PDF format (I have orcad and pads but PDF means I
    can't modify the design!). I worked on the PCI bus on AMD's Athlon and
    Athlon64/Opteron reference design MBs. (And I have seen people try to get
    away without supporting the spec correctly when it comes to the power

  16. keith

    keith Guest

    On Thu, 09 Sep 2004 10:34:01 +0000, Dana Raymond wrote:

    This isn't strictly true. There is an example in the spec for
    "loads" (each chip being one and each connector one, giving two for a card
    plugged into the bus), but the spec doesn't use these for anything other
    than an example. The timings are the critical specifications. If you can
    meet the timings, you can load the bus more heavily.
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