Confused about DDR3 memory timings and Core i5 (P55)

kidscruff · 5 Oct 2009 at 22:45

Hi, have searched and read a lot of threads, but seem to missing something vital:

Basically, thinking of running a Core i5 750 CPU on a P55 motherboard (specifically Gigabyte GA-P55M-UD2), with 4GB of DDR3 memory. Now, it seems DDR3 memory runs at a standard 1333MHz, but there are faster modules obviously - 1600MHz, etc.

The Core i5 750 CPU runs at 133MHz x20, and from what I gather, to run DDR3 at 1600, I will have to run my CPU at 160MHz, therefore overclocking it? What am I missing here? Won't the motherboard just run the CPU at 133x20 and the memory at 133x12 or something? How come this isn't just plug and play? :confused:

kidscruff · 5 Oct 2009 at 23:28

So can the RAM multiplier be x12 on the P55 (Core i5) motherboard?

Found here: http://www.anandtech.com/weblog/showpost.aspx?i=647, specifically "The 750 platform is running at DDR3-1333 with 6-6-6-18 1T timings since the 12x multiplier is not available on the i5/750"

So I can only presume that if the multiplier is maxed out at x10, then the only way to reach 1600MHz would be to overclock the CPU bus to 160MHz. Which explains all the threads on overclocking the processor.

Makes 1600 DDR3 seem more pointless unless one is overclocking the system...

kidscruff · 6 Oct 2009 at 00:38

Relevant forum: http://forums.overclockers.co.uk/showthread.php?t=18061691

After more reading, seems x10 is definitely the maximum for Core i5. Therefore i5 750 (at 133x20) must be clocked at 160x16 to enable 1600 DDR3.

Think I might go for some 1333 DDR3 at CL7 and run at default speeds, rather than plumping for 1600 DDR3 and either underclocking the memory or (somewhat) overclocking the CPU.

I suppose I could buy 1600 DDR3 with future overclocking in mind, but presume I would have to underclock this manually to 1333 (and set all the timings myself) as the XMP would just try and bump my CPU to 160Mhz.

kidscruff · 6 Oct 2009 at 00:54

good stuff! So your CPU is at 160x16, and Turbo still works, etc...?

kidscruff · 6 Oct 2009 at 10:54

Seems to depend on the motherboard as to whether the turbo option is disabled with an increased base clock (BCLK).

Also it seems that by upping the BCLK to 160MHz using the memory XMP profile, sometimes this automatically lowers the CPU multiplier to x16 in order to maintain the standard 2.6GHz, and sometimes just seems to clock the CPU to 160x20 = 3.2GHz (i.e. multiplier fixed at x20).

I understand you can always override these auto settings in the BIOS. Just interesting seeing what happens automatically.

Here's another interesting artice at AnandTech: linky - it shows the maximum overclock with Turbo enabled/disabled. For the i5 750, the 'standard' 160x20 (3.2GHz) boosts all the way to 160x27 (4.16GHz) on a single core.

Other reports on the internet (can't find them atm) seem to suggest the turbo function either disables, or only boost +1 multiplier if the BCLK is overclocked from default (133MHz).

kidscruff · 7 Oct 2009 at 09:58

True 1156 wasn't intended as an overclocker - but seems pretty happy about it anyway!

In the end I ordered some G.Skill Ripjaw 4GB (2x2GB) DDR3 PC3-12800 C7 1600MHz Dual Channel Memory for my i5 build. I figure I can run it at 1333 for now on tight timings, and bump it to 1600 whilst keeping 7-7-7-24 timings when I feel I need to boost my PC a little! Plus it's about the same price as fast 1333 Memory (Patriot or OCZ, etc)