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DDR3 1600 will only run at 1333?? G.Skill ripjaws + P7P55D

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January 13, 2010 4:22:34 PM

I have 2x2gb G.Skill ripjaws 7-8-7-24 1.6v in a Asus P7P55D and core i5. In bios, under Ai-tuning, dram frequency choices only go up to 1333mhz. Why don't I have the choice to use it at 1600 as advertised on the RAM? Any help would be appreciated. (If you can't tell by now, I'm a complete beginner with all this) Thanks!
a c 163 } Memory
January 13, 2010 5:36:22 PM

What exact model of mobo?, because on newegg.com you can find at least 10 diferent models.
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a b } Memory
January 13, 2010 5:50:28 PM

You machine's base clock is 1066, and to run at stock CPU speeds you don't need anything higher than 1066 rated RAM. OC'ing RAM will provide little performance increase.

However, you can take a stock 2.66 GHz CPU and push it up to 4.0 Ghz as changing your BCLK from 133 (2.66 GHz) to 200 (4.0 Ghz) also pushes the RAM up from 1066 to 1600.
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January 13, 2010 5:56:53 PM

saint19 said:
What exact model of mobo?, because on newegg.com you can find at least 10 diferent models.


P7P55D. no pro, no le, just P7P55D
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January 13, 2010 5:58:18 PM

JackNaylorPE said:
You machine's base clock is 1066, and to run at stock CPU speeds you don't need anything higher than 1066 rated RAM. OC'ing RAM will provide little performance increase.

However, you can take a stock 2.66 GHz CPU and push it up to 4.0 Ghz as changing your BCLK from 133 (2.66 GHz) to 200 (4.0 Ghz) also pushes the RAM up from 1066 to 1600.



so in order to increase the RAM frequency, i have to oc the cpu? The RAM is 1600 ram, so why would running it at 1600 be oc'ing it?
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January 13, 2010 6:00:49 PM

Bios options for Gigabyte p55 boards for the i5750 are 6x 8x 10x memory multi X. Same board ,bios the i7860 has 12x multi available also. Why exactly that is, I don't know. Officially the p55 supports 1066 and 1333 memory. Where as the x58 supports 800-1066 anything over is overclocking (memory controller,not usually a issue). So with your 750 at stock base clock you can get 1333 with 10x. You can raise your baseclock, but then you will be overclocking your cpu.

Your not ready to try the other way. Which would be lower your cpu multi and raise your base clock. This would allow you to run your memory at higher speed without o/c your cpu. But doing this would alter or stop turbo function.
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a b } Memory
January 13, 2010 6:08:48 PM

This is the third time in less than a week I've helped someone with this. The chip is the determining factor. I haven't heard of a P55 motherboard yet that differs in the memory speeds (via the memory multiplier) allowed based on what chip you have.

With an i5 750 you cannot run RAM any faster than 1333 with effectively OCing your computer. You can often turn on XMP in BIOS, but this changes your BCLK which also affects your CPU speed. It might, as a side effect, end up turning off functionality such as Turbo, EIST, sleep states. You can usually turn those back on but you have to specifically do it. This differs from motherboard to motherboard.

To provide a little more detail on why memory multipliers and XMP affect your CPU, we need to discuss how the RAM and CPU speeds are determined. There are 3 important settings: BCLK, Memory Multiplier, and the CPU Clock Ratio.

RAM Speed = BCLK x Memory Multiplier
CPU Speed = BCLK x CPU Clock Ratio

By default, the 1156 socket chips run with a BCLK of 133. The CPU and RAM attain different speeds by using different multipliers.

The Max Memory Multiplier that is available to the i5 750 chip is one that allows RAM to reach a speed of 1333 MHz at default BCLK. Doing the math, this is basically 10 (some boards treat it differently, but fundamentally it can be thought of as 10) because 133 BCLK x 10 Memory Multiplier = 1333 MHz.

The Max Memory Multiplier that is available to the i7 8xx chips is one that allows RAM to reach a speed of 1600 MHz at default BCLK. Doing the math, this is essentially 12 because 133 BCLK x 12 Memory Multiplier = 1600 MHz.

The i5 750 runs with a default CPU Clock Ratio of 20. This gives it it's default CPU speed of 133 BCLK x 20 CPU Clock Ratio = 2.66 GHz. When XMP is turned on for 1600 MHz RAM, the BCLK is usually changed to 160 (this could differ between boards as well). Remember, the max memory multiplier available to the i5 750 is 10, so to hit 1600 MHz the motherboard must change the BCLK to: 1600 / 10 = 160.

As a result of this BCLK change, the CPU speed changes. If the CPU Clock Ratio did not change your CPU would be OCed to 20 x 160 = 3.2 GHz. Most (if not all) motherboards would deem this too dangerous to allow when a user only flips the XMP profile. So, the boards drop the CPU Clock Ratio in response to the raised BCLK. Based on what I've seen around the forum, most boards will drop the CPU Clock Ratio to 17. This means your CPU is running at a speed of 17 * 160 = 2.72 GHz.

I don't know how Asus handles this, but Gigabyte treats this as an OC. If you have left other settings as is, they will disable Turbo, EIST, and Sleep States. These functions can be turned back on, but you have to go in and flip the settings from "Auto" to "Enabled". "Auto" means the motherboard decides whether or not to allow these functions. By setting it to "enabled" you ensure that they are always available.

Of course once you have XMP turned on and your BLCK gets bumped up to 160, Turbo will run your machine even faster. When Turbo kicks in, the CPU Clock Ratio changes. For example, with one core of an i5 750 active Turbo might raise the CPU Clock Ratio as high 24. This gives a stock CPU a max speed of 133 BCLK x 24 CPU Clock Ratio = 3.20 GHz. But with XMP on and a BCLK of 160, your new max speed is 160 x 24 = 3.84 GHz (assuming turbo raises the multiplier this high**). You'll want to test your system for stability running at these settings. Keep an eye on V and heat.


Putting this all together, at default/stock we have:
BCLK = 133
Memory Multiplier (effectively) = 10
CPU Clock Ratio = 20

RAM Speed = BCLK x Memory Multiplier = 133 x 10 = 1333 MHz
CPU Speed = BCLK x CPU Clock Ratio = 133 x 20 = 2.66 GHz

When Turbo kicks in (for example, let's say it is at it's max):
BCLK = 133
Memory Multiplier (effectively) = 10
CPU Clock Ratio = 24

RAM Speed = BCLK x Memory Multiplier = 133 x 10 = 1333 MHz
CPU Speed = BCLK x CPU Clock Ratio = 133 x 24 = 3.20 GHz


With 1600 RAM and XMP on
BCLK = 160
Memory Multiplier (effectively) = 10
CPU Clock Ratio = 17

RAM Speed = BCLK x Memory Multiplier = 160 x 10 = 1600 MHz
CPU Speed = BCLK x CPU Clock Ratio = 160 x 17 = 2.72 GHz

With 1600 RAM and XMP on, when Turbo kicks in, assuming it's turned on (for example, let's say it is at it's max):
BCLK = 160
Memory Multiplier (effectively) = 10
CPU Clock Ratio = 24**

RAM Speed = BCLK x Memory Multiplier = 160 x 10 = 1600 MHz
CPU Speed = BCLK x CPU Clock Ratio = 160 x 24 = 3.84 GHz

**I haven't tested an i5 750 with 1600 RAM and XMP and Turbo turned on to know if this is how it will actually handle the Turbo mode. If you do run in this mode, please test it and let me know if my hypothesis is correct. It's possible the turbo doesn't ramp the cpu multiplier all the way up to 24. And of course some boards may not change the BCLK to 160 and CPU multiplier to 17.


EDIT: I use XMP as an example. You could of course make the same changes manually in BIOS.
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a c 163 } Memory
January 13, 2010 6:18:56 PM

Try to set the Voltage of your memory manually in the BIOS.
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January 13, 2010 6:32:22 PM

I belive those ripjaws need to run at 1.65 volts
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January 15, 2010 3:19:52 PM

ekoostik said:
This is the third time in less than a week I've helped someone with this. The chip is the determining factor. I haven't heard of a P55 motherboard yet that differs in the memory speeds (via the memory multiplier) allowed based on what chip you have.

With an i5 750 you cannot run RAM any faster than 1333 with effectively OCing your computer. You can often turn on XMP in BIOS, but this changes your BCLK which also affects your CPU speed. It might, as a side effect, end up turning off functionality such as Turbo, EIST, sleep states. You can usually turn those back on but you have to specifically do it. This differs from motherboard to motherboard.

To provide a little more detail on why memory multipliers and XMP affect your CPU, we need to discuss how the RAM and CPU speeds are determined. There are 3 important settings: BCLK, Memory Multiplier, and the CPU Clock Ratio.

RAM Speed = BCLK x Memory Multiplier
CPU Speed = BCLK x CPU Clock Ratio

By default, the 1156 socket chips run with a BCLK of 133. The CPU and RAM attain different speeds by using different multipliers.

The Max Memory Multiplier that is available to the i5 750 chip is one that allows RAM to reach a speed of 1333 MHz at default BCLK. Doing the math, this is basically 10 (some boards treat it differently, but fundamentally it can be thought of as 10) because 133 BCLK x 10 Memory Multiplier = 1333 MHz.

The Max Memory Multiplier that is available to the i7 8xx chips is one that allows RAM to reach a speed of 1600 MHz at default BCLK. Doing the math, this is essentially 12 because 133 BCLK x 12 Memory Multiplier = 1600 MHz.

The i5 750 runs with a default CPU Clock Ratio of 20. This gives it it's default CPU speed of 133 BCLK x 20 CPU Clock Ratio = 2.66 GHz. When XMP is turned on for 1600 MHz RAM, the BCLK is usually changed to 160 (this could differ between boards as well). Remember, the max memory multiplier available to the i5 750 is 10, so to hit 1600 MHz the motherboard must change the BCLK to: 1600 / 10 = 160.

As a result of this BCLK change, the CPU speed changes. If the CPU Clock Ratio did not change your CPU would be OCed to 20 x 160 = 3.2 GHz. Most (if not all) motherboards would deem this too dangerous to allow when a user only flips the XMP profile. So, the boards drop the CPU Clock Ratio in response to the raised BCLK. Based on what I've seen around the forum, most boards will drop the CPU Clock Ratio to 17. This means your CPU is running at a speed of 17 * 160 = 2.72 GHz.

I don't know how Asus handles this, but Gigabyte treats this as an OC. If you have left other settings as is, they will disable Turbo, EIST, and Sleep States. These functions can be turned back on, but you have to go in and flip the settings from "Auto" to "Enabled". "Auto" means the motherboard decides whether or not to allow these functions. By setting it to "enabled" you ensure that they are always available.

Of course once you have XMP turned on and your BLCK gets bumped up to 160, Turbo will run your machine even faster. When Turbo kicks in, the CPU Clock Ratio changes. For example, with one core of an i5 750 active Turbo might raise the CPU Clock Ratio as high 24. This gives a stock CPU a max speed of 133 BCLK x 24 CPU Clock Ratio = 3.20 GHz. But with XMP on and a BCLK of 160, your new max speed is 160 x 24 = 3.84 GHz (assuming turbo raises the multiplier this high**). You'll want to test your system for stability running at these settings. Keep an eye on V and heat.


Putting this all together, at default/stock we have:
BCLK = 133
Memory Multiplier (effectively) = 10
CPU Clock Ratio = 20

RAM Speed = BCLK x Memory Multiplier = 133 x 10 = 1333 MHz
CPU Speed = BCLK x CPU Clock Ratio = 133 x 20 = 2.66 GHz

When Turbo kicks in (for example, let's say it is at it's max):
BCLK = 133
Memory Multiplier (effectively) = 10
CPU Clock Ratio = 24

RAM Speed = BCLK x Memory Multiplier = 133 x 10 = 1333 MHz
CPU Speed = BCLK x CPU Clock Ratio = 133 x 24 = 3.20 GHz


With 1600 RAM and XMP on
BCLK = 160
Memory Multiplier (effectively) = 10
CPU Clock Ratio = 17

RAM Speed = BCLK x Memory Multiplier = 160 x 10 = 1600 MHz
CPU Speed = BCLK x CPU Clock Ratio = 160 x 17 = 2.72 GHz

With 1600 RAM and XMP on, when Turbo kicks in, assuming it's turned on (for example, let's say it is at it's max):
BCLK = 160
Memory Multiplier (effectively) = 10
CPU Clock Ratio = 24**

RAM Speed = BCLK x Memory Multiplier = 160 x 10 = 1600 MHz
CPU Speed = BCLK x CPU Clock Ratio = 160 x 24 = 3.84 GHz

**I haven't tested an i5 750 with 1600 RAM and XMP and Turbo turned on to know if this is how it will actually handle the Turbo mode. If you do run in this mode, please test it and let me know if my hypothesis is correct. It's possible the turbo doesn't ramp the cpu multiplier all the way up to 24. And of course some boards may not change the BCLK to 160 and CPU multiplier to 17.


EDIT: I use XMP as an example. You could of course make the same changes manually in BIOS.


thanks man, that was really helpful. When I do try it out, ill let you know the results.
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a b } Memory
January 15, 2010 3:59:30 PM

You're welcome. Good luck!
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March 12, 2010 8:02:13 PM

I Got i5 750 on Biostar t5 xe cfx-sli, and ddr3 1600 GSkill ripjaws memory. i have tried to set BLCK to 160 (3.2Ghz) and it is unstable with a turbo on and a stock voltage.
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August 19, 2010 9:22:19 PM

ekg84 said:
I Got i5 750 on Biostar t5 xe cfx-sli, and ddr3 1600 GSkill ripjaws memory. i have tried to set BLCK to 160 (3.2Ghz) and it is unstable with a turbo on and a stock voltage.


I also have the same setup, but with an i5-760. When trying to overclock the FSB to 160 and achieve 160x10 - 1600 mhz memory clock, system is unstable when running prime95. Does anyone have any suggestions for getting this hardware combination stable?
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a b } Memory
August 20, 2010 2:26:50 AM

Seems to be alot of waking old threads from the dead going on around here. I assume you know when raising the bclk to 160 you are overclocking your CPU too. So there will be some stability issues until you get the right level of CPU voltage and RAM V and timings, and possibly other settings.

My suggestion, start a new thread, describe what you are trying to do, list out all of your components - ALL of them even down to the OS is usually needed (admittedly in your particular case it is probably not), point being list more details than you think are important and include models numbers and links - then list what settings you have tried and what you have observed and the problem you're encountering. Be specific and detailed.

By starting a new thread you're more likely to get help from people actively monitoring the forums. If you come across a thread that looks similar (and by the way, great job in searching the threads, many people won't even do that, so don't take this as a negative response, thank you for doing some research) - if you find a thread that looks similar but is more than a month or two old PM the contributors and send them a link to your thread, ask them if they would mind giving you a hand. Hopefully you'll get their attention but if not you'll at least have others helping out who wouldn't normally want to look through an old and very long thread.
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July 11, 2012 12:16:41 AM

Hello! This is my first post on a forum... Ever! However, I've oc'd my AMD Phenom II 955be and my graphics card and learned how to build my rig from this forum. So thank you all so much for contributing! I'm having the same problem. First, I lost the manual for my motherboard so I'm having a hard time figuring out what exactly I have. It's a Gigabyte GA durable 3 something I know that much. I upgraded my 2x2 corsair dominator 1333 to vengeance 2x4 1600. My bios is still reading 1333. Am I having the same problem as this guy?

Thanks again.
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