For Those Who Don't Know What SPD memory speed is for

Discussion in 'Overclocking & Cooling' started by HAL_404, Jan 4, 2020.

  1. HAL_404

    HAL_404 Gawd

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    this is a lengthy post; might want to first get a cup of Java or a beer ...

    I read not too few comments online about how AMD MoBo's were having issues regarding MFG/Retailer posted mem speeds vs how it would post in the MoBo's BIOS via Auto settings and that when XMP was enabled ... why it still didn't show the MFG's/Retailer's posted mem specs.

    Today I began to question why, when I bought DDR4 3000 memory with advertised timings of 15 - 17 - 17 - 35 why it was that it wound up posting in the Mobo's BIOS as DDR4 2166 with timings of 16 - 21 - 22 - 50

    So I went to the memory MFG's website and looked up the specs and it showed as follows:

    SPD Latency 15-15-15-36
    SPD Speed 2133MHz
    Tested Latency 15-17-17-35
    Tested Speed 3000MHz

    The Retailer's website only showed the tested numbers so then I thought, "ok, why did the memory MFG post two different sets of specs for the same memory module?"

    Then I decided to look up what SPD meant and lo and behold I found the following description:

    "When a computer is booted (started), serial presence detect (SPD) is information stored in an electrically erasable programmable read-only memory (EEPROM) chip on a synchronous dynamic random access memory (SDRAM) memory module that tells the basic input/output system (BIOS) the module's size, data width, speed, and voltage. The BIOS uses this information to configure the memory properly for maximum reliability and performance."

    I had to read that definition a couple times before it would sink in and even then, I continue to disagree with the "maximum performance" part https://whatis.techtarget.com/definition/serial-presence-detect-SPD but I digress here ...

    And so, I began to learn that memory speed/timings on AMD MoBo's (recent one's anyways) are automatically set to the SPD values hence the reason we will see an XMP On/Off button in the MoBo's BIOS settings (recent boards anyways) so that the TESTED values will be applied for those of us who are "enthusiast" gamers

    But the XMP button doesn't work right, sets my 3000 mem to 2666 and that won't do so rather than presume the MoBo was defective I decided to manually selected DDR4 3000 from a drop-down list in BIOS under Advanced Memory Settings and then set the timings to 15 - 17 - 17 - 35 which was a chore since the MSI BIOS showed two tRCD settings (tRCDRD and tRCDWR) ... geeesh :barefoot:

    Why they make doing this so difficult for the average gamer?

    So my question then became, "why bother putting a XMP button in the BIOS when 'They' know it's not going to work as expected?" But yet once again I digress here ...

    And if that wasn't enough to cause aggravation for the average gamer (no, not the digression part) ... then there's the tRC setting that doesn't ever seem to get mentioned anywhere in the specs ...

    for those still reading and in need to know what tRC is:

    " The minimum time interval between successive ACTIVE commands to the same bank is defined by tRC where tRC = tRAS + tRP "

    https://www.techpowerup.com/articles/64

    you'll likely need to hunt for the tRC setting under the BIOS memory Advanced settings to find it ... at least I had to

    And so, if you bought or are considering buying an AMD MoBo, you are now a well informed rig builder as to why XMP likely won't work right for you either and why you'll need to manually set the Mem specs for yourself in BIOS

    example:

    tCL ..... tRCD ..... tRP ..... tRAS
    15 ......... 17 ........ 17 ........ 35

    and don't forget to set tRC ;)
     
    Last edited: Jan 4, 2020
  2. JosiahBradley

    JosiahBradley [H]ard|Gawd

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    Yeah it's going to be fun with my kit as XMP sets the voltage .3v too low for the rated speed. It's trying to run 3800 CL14 at 1.2 volts. Not even sure what other subtimings it set. I'm going to use the ryzen dram calculator to set everything anyhow. Thanks for the post.
     
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  3. Jamie Marsala

    Jamie Marsala Limp Gawd

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    I found the same issue on my MSI X570 board. Set the XMP on and sure it says all the timings in the profile and the speed, but it never sets the memory to that speed, in my case 3600. I manually set it and it works as expected, using the drop down, my board also set the correct timings in Auto mode. I have it bumped to 3800 with the Fabric at 1900 now though.
     
  4. kamikazi

    kamikazi Limp Gawd

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    My memory is no where close to running at XMP. If I enable XMP, it will not even post and requires a CMOS reset. It needs 1.43v to do 3733. XMP is 1.35 at 4000. Not that I would want to try to run 4000 with asynchronus fclk, but it doesn't work anyway. Ryzen DRAM calculator settings don't work either unless I make a bunch of changes. The RAM is B Die, but not on the QVL, so I guess I can't complain.
     
  5. Borgschulze

    Borgschulze 2[H]4U

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    Neat, I set XMP and it runs fine, and applies the settings correctly.

    Ryzen 3600X with G.Skill Trident Z Neo... 3600mhz on an X570 board.
     
  6. kirbyrj

    kirbyrj [H]ard as it Gets

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    Well, for starters, I would check to see whether or not gear down mode is enabled since you're trying to run an odd numbered CAS latency. I'd run it at either 14 or 16 instead of 15.