Intel Clarkdale CPU Overclocking With IGP Enabled - Reaching High BCLK

Overclocking/OC-Team.be by massman @ 2010-02-02

Since the launch of the new CPUs from Intel, nicknamed Clarkdale, we have seen a fair share of overclocking attempts; some people even breaking world records, pushing the a Clarkdale over 7Ghz. We have read reports from overclockers who had issues with pushing the BCLK higher when the IGP was enabled. In this article we will explore possible solutions to help circumvent this limit, so you can enjoy high BCLK while using the IGP.

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The hypothesis

The hypothesis

We have been mailing back and forth with Gigabyte, because in our initial testing 200MHz BCLK with IGP enabled did not work either. In fact, we were, as well as Anandtech, stuck at around 165MHz BCLK using the configuration as stated on the previous page. Although the initial results were kind of a set back, we continued our research and with the help of Gigabyte finally managed to get the system going at higher BCLK frequencies. However, instead of being satisfied that we can make a screenshot of a stable configuration ... we want to find the exact cause of the problem and an appropriate solution for it. So what's the problem?

As you can imagine, the problem is located at the level of the IGP frequency. Although it can be overclocked fairly well on air cooling - 1.1GHz is not impossible - it does impose a limitation when increasing the BCLK frequency to roughly 165MHz. And that's perfectly understandable, as long as you keep in mind that the IGP frequency is, as well as any other frequency related the CPU, relative to the BCLK frequency. To solve this IGP frequency issue, many mainboard manufacturers have already implemented a bios option that allows users to change the IGP clock frequency. On the mainboard we've tested, this bios issue can be found in the overclocking section.

Madshrimps (c)


Now, against all expectations of both the end-user and, apparently, the manufacturer, it seems that this bios option does NOT set the graphics core frequency to a fixed value. What it does is very simple: it changes the IGP base frequency. As none of the current applications allow me to correctly measure the graphics core frequency, I have to test this theory using a less elegant methodology, but ... at least one that works.

The two key words of this methodology are "performance scaling". During my test sessions, I've used the 3DMark03 benchmark to see how much performance was delivered by the IGP frequency. Since the performance level of this 3D technology is quite low, the influence of CPU and QPI frequency are neglegible, if kept within small margin, when comparing results. Here is the list of results:

FORMAT: "BCLK/IGPclk= score"

  • 200/500 = 5166
  • 200/666 = 6543
  • 200/700 = 6989
  • 150/auto = 6567
  • 165/auto = 7116


  • Given that I'm working with a Core i5 661, auto in this case equals "900MHz". The scaling is not entirely right if we only consider the frequency set in bios, so we have to come up with a different formula to calculate the IGP frequency. After a bit of research, we came up with this one:

    Madshrimps (c)


    Using this formula, we can recalculate the resulting IGP frequency

    FORMAT: "BCLK/IGPclk ~ Real IGPclk = score"

  • 200/500 ~ 750MHz = 5166
  • 200/666 ~ 1000MHz = 6543
  • 150/auto ~ 1015MHz = 6567
  • 200/700 ~ 1050MHz = 6989
  • 165/auto ~ 1115MHz = 7116


  • As you can see, the performance scaling result match the IGP clock frequency or, to make it a bit more scientific, the data fits the hypothesis. For argument's sake, let's try it the other way around as well: I make a prediction based on the hypothesis, run the benchmark and see if the prediction is correct. I ran these the following BCLK/IGPclk combinations:

  • 154/626 = 4892
  • 160/600 = 5110
  • 166/578 = 4958
  • 180/532 = 5061
  • 182/527 = 4851
  • 190/505 = 5040
  • 200/480 = 4817
  • 210/456 = 4717
  • 220/435 = 4943


  • Madshrimps (c) Madshrimps (c) Madshrimps (c) Madshrimps (c) Madshrimps (c)
    (Clickable - Screenshots are a selection of the results and cannot be analyzed separately)


    All the same resulting IGP clk of ~ 721MHz assuming the above formula is correct, all pretty much the same frequency. I will add the screenshots of the results to the forum thread of this article. Note that a 900MHz IGP will score roughly 6000 to 6200 points ... the variance as seen in above list is not significant enough to disqualify the hypothesis.
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    Comment from Massman @ 2010/02/02
    Attaching the screenshots of the "721MHz frequency testing". Warning ... a lot!
    Comment from Massman @ 2010/02/02
    As far as I can understand how the IGP frequency is made, the IGPbase frequency is not function of BCLK, but of BCLK/4.

    So:

    IGPbase = 133*multiplier/4

    or

    IGPbase = 33*multiplier.

    The multiplier itself is one value ... officially.
    Comment from Massman @ 2010/02/02
    Straight from Intel datasheet:

    //edit: added second one.
    Comment from geoffrey @ 2010/02/02
    011000binary makes 24decimal, 24x33 gives 800
    Comment from Massman @ 2010/02/03
    It seems that the clock frequency of the Intel GMA can vary between 166MHz and 900MHz. For desktop CPU's, that's 733 and 900 only, for mobile the igp frequency is automatically decreased when turbo mode is enabled and you're running high load CPU applications. So:

    166 = 33 x 5 = 000101b
    200 = 33 x 6 = 000110b
    233 = 33 x 7 = 000111b
    266 = 33 x 8 = 001000b
    300 = 33 x 9 = 001001b
    333 = 33 x 10 = 001010b
    366 = 33 x 11 = 001011b
    400 = 33 x 12 = 001100b
    433 = 33 x 13 = 001101b
    466 = 33 x 14 = 001110b
    500 = 33 x 15 = 001111b
    533 = 33 x 16 = 010000b
    566 = 33 x 17 = 010001b
    600 = 33 x 18 = 010010b
    633 = 33 x 19 = 010011b
    666 = 33 x 20 = 010100b
    700 = 33 x 21 = 010101b
    733 = 33 x 22 = 010110b - DESKTOP
    766 = 33 x 23 = 010111b
    800 = 33 x 24 = 011000b
    833 = 33 x 25 = 011001b
    866 = 33 x 26 = 011010b
    900 = 33 x 27 = 011011b - DESKTOP
    Comment from Massman @ 2010/02/03
    IGPclk = IGPbase x (BCLKset / 133) MHz

    Now: IGPbase = (BCLKset / 4) x IGPmp

    Thus: IGPclk = (BCLKset / 4) x IGPmp x (BCLKset / 133) MHz

    Or: IGPclk = (BCLKset² x IGPmp) / 533 MHz

    That looks weird
    Comment from Massman @ 2010/02/03
    When looking a the desktop datasheets, I'm a bit confused, though.
    Comment from Massman @ 2010/02/05
    A bit more testing: 100MHz and 200MHz BCLK:

    1200 * (100/133) = 902MHz
    600 * (200/133) = 902MHz

    Performance: 5525 vs 5435
    Comment from geoffrey @ 2010/02/05
    Don't make clock dividers over-complicated
    PS home tomorrow?
    Comment from Massman @ 2010/02/05
    Yes.
    Comment from thorgal @ 2010/02/22
    Comment from rickss69 @ 2010/03/14
    ***
    Comment from rickss69 @ 2010/03/14
    ***

     

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