9600GT core clock mystery seem to be revealed

Discussion in 'RivaTuner Advanced Discussion forum' started by Unwinder, Feb 29, 2008.

  1. Unwinder

    Unwinder Moderator Staff Member

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    Gentlemen, it looks like we can finally close 9600GT core clock mystery, which I’ve been brainstorming with my friends during the last 2 weeks. The mystery was not visible to public, however almost all reviewers noticed it during monitoring 9600GT clocks in realtime. Unfortunately, it was not exposed in majority of 9600 reviews, but TechPowerUp’s Zotac 9600 GT review is a good example of review which focused attention to it (please refer to “Wrong core clock?”):

    http://www.techpowerup.com/reviews/Zotac/GeForce_9600_GT_Amp_Edition/24.html

    Summarizing:

    NVIDIA insisted that they use 25MHz crystal for 9600GT’s core clock generation, which sounded a bit suspicious for us because it is simply not logical to use two different crystals on the same PCB and furthermore we couldn’t locate 25MHz crystal on the PCB. NVIDIA also refused to comment our questions about lack of 25MHz crystal on the PCB.
    However, yesterday I’ve found really interesting report in the forum, which pushed me in right direction and shed some light on the mystery. One of 9600GT owners (kudos to Gljukats @ overclockers.ru) reported that his 9600GT is really sensitive to PCI-E bus clock tweaking and even minor PCI-E clock increase increases 9600GT performance in 3D benchmarks (which are normally not limited by PCI-E bus bandwidth). This allows me to make the following conclusion: there is no fixed 25MHz reference clock and NVIDIA uses quarter of PCI-E bus clock (100MHz / 4 = 25MHz) as reference clock for core frequency generator. I’m almost 100% certain that my assumption is correct, however I still don’t have 9600GT installed and cannot verify it myself. Anyway, I hope that I’ll be able to do it really soon.
    So 9600GT owners should keep in mind that GPU clock is linked with PCI-E clock on these cards. So for example by overclocking the bus from 100MHz to 120MHz you also overclock your 9600 GT GPU by 20%. And the most tricky thing: you won’t be able to see clock frequency changes either in NV driver or in RivaTuner or any other software, because NVIDIA always uses fixed 25MHz crystal clock in the calculations.
     
  2. Rammstein

    Rammstein Master Guru

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    GPU:
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    Sounds intresting, well done. I hope nvidia fixes their GPU Mhz reading in ntune aswell.
    Does this mean the release of 2.07 comes soon?
    So increasing the PCI-E MHz does only higher the GPU core and Memory will still be at 27Mhz as before, right?
     
    Last edited: Feb 29, 2008
  3. Unwinder

    Unwinder Moderator Staff Member

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    Don't think that they'll change it. Reading true GPU clock in this case means that the driver will need to read PCI-E clock directly. That's a personal codepath for each mainboard chipset and even many mainboard models, using external clock generators. Display driver is not a place to implement such things. Most likely they will always suppose that the clock is default and use fixed 25MHz for core clock calculation. I'll also do the same in RT's hardware monitor, I physically won't be able to support dozens on mainboard chipsets and models because RT is not oriented on low-level mainboards support.

    No. 2.07 will be launched as soon as my 9600GT sample arrives. I'm already waiting for it during almost 2 weeks.

    Yes.
     
    Last edited: Feb 29, 2008
  4. Unwinder

    Unwinder Moderator Staff Member

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  5. cowie

    cowie Ancient Guru

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    why hide that from end users?
    and do you think just referance cards like this? and about 98's i wonder?
    if so i have to use my marvel controler on my dfi board because other does not like to allow high pci-e clocking
    weird
    but thank to you guys for sheding the light on this
     
  6. Unwinder

    Unwinder Moderator Staff Member

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    I guess that they tried to hide it from the competitors, not from the end users. Just like it was with independent domains clocking a few years ago, which we've discovered after 7800GTX announce.

    I'm pretty sure that will apply to both reference design and non-reference design 9600's. Cannot say anything about 9800 until seeing a sample.

    I'm afraid that there is some misunderstanding. What makes you think so?
     
  7. cowie

    cowie Ancient Guru

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    because i get crased and try to clock the crap out of everything sir.
    sry
    thanks for your time
     
  8. bug77

    bug77 Banned

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    Really not a big deal, imho, because anyone who knows anything about overclocking locks the PCI and PCIe frequencies first. Good to know how it actually works nevertheless.
     
  9. ViperJohn

    ViperJohn Member Guru

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    I noted the same thing in my tests with the card. The core clocks generator master reference frequency on the 9600GT is the PCIe bus frequency divided by 4. This would normally be 100Mhz /4 = 25Mhz except with the NV new 780i MB.

    NV's reference 780i has a default PCIe bus frequency of 125Mhz so a 9600GT stuffed in a PCI slot of a reference 780i is going to have a core core 25% higher than in a 100Mhz default PCIe bus frequency motherboard.

    I find it hard to believe that NV didn't have that tid bit in mind when they tied the 9600GT core clock generator master reference frequency to the PCIe bus frequency!!!

    Viper
     
  10. Hilbert Hagedoorn

    Hilbert Hagedoorn Don Vito Corleone Staff Member

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    Sorry but, exactly where do you base that info on ?

    I just checked on my 780i (eVGA Ref) and it's 100 MHz, if you check any review you'll notice that all of them show a 100 Mhz reference speed.
     

  11. == x86 ==

    == x86 == Member Guru

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    Yeah dude, where did you get that info? My 780i Mainboard is on the way right now... just make sure I don't get a wrong board. 125Mhz PCI-E Bus, I think that's too high it can damage the graphic card right?
     
  12. ViperJohn

    ViperJohn Member Guru

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    From my eVGA reference 780i. Just fire up ClockGen.

    [​IMG]

    It is not Link Boost related as I get 125Mhz with either NV or ATI cards inserted.

    The middle 16x PCIe 1.0x slot is 100Mhz default and is adjustable in system bios. The top and bottom 16x PCIe 2.0 slots are 125Mhz and not adjustable.

    Possibly what NV was hoping for???


    Edit - Let me clarify this a little more. The PCIe 1.x slots coming off the MCP are running a normal 100Mhz. It is the top and bottom 16x PCIe 2.0 slots coming off the NF200 chip that are running 125Mhz.

    Viper
     
    Last edited: Mar 2, 2008
  13. ViperJohn

    ViperJohn Member Guru

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    No.

    Viper
     
  14. == x86 ==

    == x86 == Member Guru

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    GPU:
    Geforce 8800GT 512MB
    Just a little confuse here...
    If it's come by default 125Mhz, it's okay to attached my 8800GT in there? Because I have some bad experience with playing PCI-E freq, my previous 8600GT became completely unused cause I set the PCI-E to high (120Mhz). Or maybe the default value for PCI-E 1.0 is 100Mhz while the newest PCI-E 2.0 is 125Mhz? and my 8800GT is capable of PCI-E 2.0 so it's okay to attached it right there... Any help would appreciated. Thanks in advanced
     
  15. ViperJohn

    ViperJohn Member Guru

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    You won't have any problems.

    Viper
     

  16. == x86 ==

    == x86 == Member Guru

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    GPU:
    Geforce 8800GT 512MB
    so it's true that the newest PCI-E 2.0 is rated at 125Mhz?
     
  17. ViperJohn

    ViperJohn Member Guru

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    Across all motherboards no it would 100Mhz I believe for PCIe 2.0 slots just like PCIe 1.x slots.

    On the 780i something is running 125Mhz related to the PCIe 2.0 slots and ClockGen is picking that up. Now weather the PCIe bus frequency the 2.0 slot itself and/or the frequency of the link between the NF200 chip and the SPP on the 780i is unanswered.

    Viper
     

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