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I own a Dell Vostro 2520 which came with an Intel Core i3 processor and 2GB RAM. I never noticed the RAM frequency before adding RAM. I thought of adding 2GB RAM as my laptop had a second unused slot. After adding when I checked the config in BIOS, it showed that my RAM frequency is 1600 Mhz. Both of my RAM are of 1600 Mhz. But Speccy shows that my DRAM frequency is 665 Mhz. I know we have to multiply it by 2 because of Double Data Rate. But again it's lower than 1600 Mhz. Why is it so? Is it because of differing RAM voltages (1.5 V and 1.35 V)?

  • Why down vote? Please explain. – kakkarot Oct 5 '15 at 10:56
  • Dual channel mode has nothing to do with the frequency. I suspect that you wanted to type "But Speccy shows that my DRAM frequency is 665 MHz. I know we have to multiply it by 2 since it is DOUBLE data Rate memory (DDR) which sort of behaves as if it is running at twice the frequency, even though it really isn't". – Hennes Oct 5 '15 at 13:11
  • Edited. But that doesn't explain the down vote. – kakkarot Oct 5 '15 at 16:03
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Most likely it's 666Mhz (1333MT/s). Some processors are not capable of 800Mhz (1600MT/s). The 2nd gen i3 processors in the Vostro 2520 (and most other 2nd Gen core i CPUs) do not support 1600MT/s, only 1333MT/s.

An example, the i3 2328M from a Vostro 2520:

http://ark.intel.com/products/70927/Intel-Core-i3-2328M-Processor-3M-Cache-2_20-GHz

Memory Types DDR3 1066/1333

Some 3rd generation and later CPUs support 1600MT/s though.

Also note the "multiply by two" has nothing to do with "dual channel mode". The multiplying by two occurs because it's DDR RAM. DDR stands for double data rate.

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Its highest common frequency. If you don't pair the DIMMs they'll operate at the best they can both do. Difference between 1330 and 1600 will be of very little impact unless you're doing really memory intensive tasks.

  • Both of my RAM sticks are of 1600 Mhz. So it must be 1600 right? – kakkarot Oct 5 '15 at 10:12
  • The memory controller will try to find the best configuration. It cant clock them both at 1600 for some reason. It may be timings. This is why you're advised to buy DIMMs in pairs. The fact that they're different voltages is one thing. If they're operating at 1.35-1.4v then the 1.5v stick may be underpowered and thus running slowly. – Linef4ult Oct 5 '15 at 10:20
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Most memory controllers can only drive a limited number of memory ranks at a certain speed. The most common configuration for consumer computers at this time is:

Memory controller
  |        |
  |        _____ Slot 1a ____ slot 1b
  | 
  ______________ Slot 2a ____ slot 2b   

That is four memory sockets, and up to two channels.

Typically your memory controller can drive 2 ranks of memory at max speed, and four ranks at reduced speed.


Let's map that to your situation:

Before:
Slot 1a filled with a dual rank DIMM.
Slot 2a filled with a dual rank DIMM.

These ran at 800MHz (which is reported as 1600MHz due to marketing)

Now you add two more DIMMs. Suddenly the one memory controller connection has to drive both connectors (slot 1a and slot 1b). It can not do this at the same speed and falls back to 667MHz.

Nowhere does dual channel come into the picture, nor does it have any influence.

Why is it so? Is it because of differing RAM voltages (1.5 V and 1.35 V)?

Most likely it is as stated above. Most memory controllers can drive a limited number of ranks at a certain speed. Add more and they need to slow down. (Note that the typical number of ranks per DIMM is 2, though there exist single rank DIMMs and quad rank DIMMs. The latter are often used in high density servers.)

Lastly it is quite possible that some of your memory is only rated for 800MHz at 1.5 volt. When you put in both the 1.35v memory and the 1.5v memory you got two options:

  1. Feed 1.5v to both (probably feeding more than specced to one pair of DIMMs)
  2. Feed 1.35v to both. Quite a lot of 1.5v memory can work on the lower voltage, but at reduced speed.

Either or both of the reasons explained above is likely to dropped your memory speed.

So much for a nice generic answer. Now for your specific question: You had two SoDIMM slots. The second reason is likely your cause. If you have a firmware (BIOS or UEFI) with lots of options you might force both SoDIMMs to 1.5 volt or manually set the memory speed. Most laptops are quite limited in what you can configure, though.

  • Rather the opposite, 1.5v memory will never be forced to run at 1.35v, only the other way round. By design 1.35v memory must also be able to run at 1.5v therefore all memory will run at 1.5v when 1.35v and 1.5v sticks are mixed. – qasdfdsaq Oct 5 '15 at 14:54
  • I have three 1.5v and three 1.35v DIMMS in my desktop (granted, not a laptop and full sized DIMMs rather than SODIMMS) which currently all run at 1.35. In highsight the 1.5 is rather for both 1.35 (at normal speed) and at 1.5 (with higher speeds). I might have manually set them both to 1.35. Will need to check when I reboot. – Hennes Oct 5 '15 at 15:09
  • Either way, the standards require 1.35v RAM to also be fully compatible with running at 1.5v, it doesn't require the other way round - technically some call DDR3L "dual voltage 1.35/1.5v" rather than just "low voltage" to emphasize this: dell.com/support/article/us/en/19/SLN153768/EN Some 1.5v RAM can work at 1.35v (or lower) but isn't certified to do so, and needless to say any BIOS should never default to a voltage that a chip does not report itself as capable of. – qasdfdsaq Oct 5 '15 at 15:11

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