EggXpert

This guide lists an example of memory timings and in which order they come in.
It should act as quick references for those of you wanting to check your memory timings via BIOS to ensure that your timings are set correctly.  You should always ensure that your memory is set to rated timings for optimal performance unless you are trying to overclock.

NOTE: It is in the order of CAS-tRCD-tRP-tRAS. Some motherboard manufacturers list their timings in the order of CAS-tRCD-tRAS-tRP.  You'll usually be able to tell since the tRAS value is usually the highest.

The example timings we’ll use are 5-4-4-12

• CAS (tCL) Timing - (Column Address Strobe/Select)
  5-4-4-12
  The bold "5" is the CAS timing.

• tRCD Timing - RAS to CAS delay.  (Row Address Strobe/Select to Column Address Strobe/Select)
  5-4-4-12    
  The bold "4" is the tRCD timing.

• tRP Timing - Row Precharge Time
  5-4-4-12    
  The bold "4" is the tRP timing.

• tRAS Timing - Min RAS Active Time
  5-4-4-12    
  The bold "12" is the tRAS timing.

There are many excellent, in depth guides on memory timings at websites such as www dot overclocking dot net.  But I'd rather see some smart enthusiasts from these forums make their own guides!  Feel free to post any questions, comments or suggestions.

And remember, don't play with fire!

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Nice.

Also if you are going to run four sticks of memory, you may need to loosen up your timings (EG: from 5-4-4-12 1T to 5-4-5-15 2T) for it to run stable.

You should also test your memory using something like Memtest86/ freeware before you try to use your new computer just to make sure your timings are correct, if it comes up with errors, try adjusting your latencies until they are gone. If you can't seem to get rid of the errors, it's possible you received a brand new stick of bad ram, then test them one at a time if necessary to see what one is bad.

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Thought I might contribute an explanation of timings

Primary RAM Latencies
 As you can see, there are lots of latencies that contribute to the amount of time it takes to read the RAM.
The most important latencies to be aware of during standard RAM operation are the following:

• CAS Latency is the delay, in clock cycles, between when the READ command is issued and when the data on the DQ pins is valid. Standard values for DDR memory are 2 and 2.5 clock cycles. Values of 3 and/or 1.5 clock cycles are available in some systems and are supported by some (but not all) RAMs. Note that a CAS latency of 2 cycles means that the data is valid on the rising edge of the second clock after READ is issued; a latency of 2.5 cycles means data will be available on the falling edge of the second clock following READ.

• RAS-to-CAS Delay is known as tRCD. It is the delay, in clock cycles, from when the ACTIVE command is issued to when a READ or a WRITE command can be issued. Is generally set to either 2, 3, or 4 clock cycles.

• RAS Precharge is also known as tRP. It is the delay, in clock cycles, from when the PRECHARGE command is issued to when the ACTIVE command can be issued for another row. Common settings for RAS Precharge are 2, 3, or 4 clock cycles.
ACTIVECommandtRCDRAS to CAS delayREAD and/orWRITEAdditionalREADs and WRITEsPRECHARGECommandtRPRAS PrechargeNext ACTIVECommandThe READ command cannot follow the ACTIVE command until tRCD cycles have elapsed.Data is not valid until one CAS Latency after the READ command (unless the READ immediately follows a previous READ)Once the PRECHARGE command has been issued, a new ACTIVE command cannot be issued until after tRP cycles.CAS LatencyDataValid
Figure 2. RAM Operation, including latencies
March, 2005 Page 4

• tRAS equals minimum ACTIVE to PRECHARGE delay. Once an ACTIVE command is issued for a given row, a PRECHARGE command cannot be issued for the row until tRAS has elapsed. tRAS is measured in clock cycles, and typical values are generally somewhere in the neighborhood of 5 to 10 clock cycles.
There is one more latency that we must be aware of, commonly known as Command Rate. Command Rate is the delay, in clock cycles, between when the CS# signal is activated and when any command (ACTIVE, for example) can be issued to the RAM. Common values for command rate are either 1 or 2 clock cycles.
Module latencies are often expressed as a combination of these values. The sequence we will use in this paper is CL-tRCD-tRP-tRAS-Command. So, a module with the following designation:
PC3200 2-3-4-5-1T
would have a clock rate of 200 MHz, CAS latency of 2 cycles, RAS-to-CAS delay of three cycles, RAS Precharge of four cycles, ACTIVE to PRECHARGE of five cycles, and a Command Rate of one cycle.

(borrowed this from Corsair) 

Edit: Also added 373282 

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GSKILL:

I hope this article helps

http://www.tomshardware.com/forum/176007-30-dram-command-rate-explained 

Nice link.

Edit: re-linked it 

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arsonic,u r so nice giving us the link

Nicod3mus:

Question: I want four gigs of ram in my system, would it be better to get 4 1 gig sticks or 2 2 gig sticks. Is there any pros/cons in either combination?

Glad you asked.  I would suggest you go 2x2GB.  Less sticks mean less chances of sticks with errors.  Plus you can run dual channel with two sticks, BONUS!!  Also, two sticks generate less heat than four sticks.

Enjoy your 4GB, i have 4GB too, its so sweeeeet.

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Thanks for your advice Arsonic, I'll take your advice and get 2 2gig sticks.

Back in the old days, if the clock timings for the memory were faster than that of the motherboard, then that would be OK, as the slowest element was the motherboard and the memory would work.  Is this still the case?  For instance a 4-4-4-12 should be able to run in motherboards at 5-5-5-15 if these are actual timings in microseconds or even clock cycles (smaller is better).. 

Just looked at Wintek products page though, and because it seems that 5-5-5-15 is associated with faster memory (DDR2 1000) than 4-4-4-12 (DDR2 800).  How longer times for the cycles gets associated with faster memories is beyond me.

Any help would be appreciated.

.   . 

What is important has changed from the old days.  I did a study awhile back that looks at some of the variations, see link. 

Regarding 800 versus 1000, it is just that they loosen the timings to get the extra speed --- they-re most likely using the same chips with some binning.  If you look at the link above you'll see the comparison.
 

Now on Main Board Standard for ram speed,

Many main boards claim that there Standard speed is say DDR2 1066. Please realize that MOST RAM moduals will still default to DDR2 800 speeds, (yes even on DDR2 1200 standard boards) anything over DDR2 800 speed is considered an overclocked speed and you will have to achieve this by setting the timings, voltages and MHz in the BIOS manually.

Be it that this is considered an overclocked speed, you can not expect ALL RAM moduals to reach there rated speed on every main board in the market (Especially the cheeper ones, lol).

So just because the STANDARD is say 1066 ram does not mean you can not run 800 or 1200 MHz ram. And if you put anything in higher than 800 MHz ram, you will have to clock the main board up to the speed of that RAM.

Also Using SLI ready memory (EPP memory) on a main board capable of using it, I have noticed that the ram will not run at recommended speeds and voltages unless you enable SLI ready memory (EPP) and set it to expert, then it will default to the speeds that your ram is designed for. from there you can OC it or leave it at its stock settings.     (on some main boards)

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RowenaDhu32:

Read through a lot of info, but still none of it tells exactly how to do the reset or how to loosen timing.  Who can tell???  Nice to have some of the guru's do a go-by for all of us nugs.   i would personally love it.  Want to build my own, but have to settle for just upgrading right now. 

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I.B.O.T.L

Arsonic, thank you for the informative article.

Nicod3mus asked about 4 1 gig vs 2 2gig sticks.   Following that theme a bit, if I am going to run Windows XP /SP3 should I get 2 x 2gig or 3 x 1gig (since it will only be able to read around 3 gigs.

Or should I uninstall SP/2/3 and stick with 2 x 2gigs?

Thanks

There will be differences, but RAM overclocks don't affect performace nearly as much as CPU or GPU overclock

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