Latencies in DDR3

[Axonn]

Hi guys & girls ::- ).

Could you link me to a good article explaining latencies in DDR 3?

Is 9-9-9-27 better than 7-7-7-20? I thought not. I thought the lower the latency the better. But then I found that the 7-7-7-20 module is cheaper ::- /.

DDR 3 latencies also seem to be greater than DDR 2. *sigh*. Maybe somebody very knowledgeable around here can explain all this latency stuff in a few short sentences? ::- ).

Thank you!

P.S.: in the meantime, I will read this area's "Sticky" post with Memory FAQ, but it is pretty dense. Also I hope you'll be able to clear me up on that latency issue I described above.

 

[Axonn]

Hm. Editing this message is not permitted. That's what I'm told when I try normal edit. Quick edit worked. Once. No more after that. *sigh*.

Wanted to add that the Sticky doesn't seem to cover DDR 3.

 

[T_T]

Axonn, if comparing just the latencies, then yes - the lower, the better. However, everyone knows that RAM quality isn't measured by just latency; and the price of RAM should never be a means of quality measurement.

While the typical latencies for DDR2 were 5-5-5-15, the standard latencies for the DDR3 are 7-7-7-20 for DDR3-1066 and 7-7-7-24 for DDR3-1333.

DDR3 latencies are numerically higher because the I/O bus clock cycles by which they are measured are shorter; the actual time interval is similar to DDR2 latencies (around 10 ns). As with earlier memory generations, faster DDR3 memory became available after the release of the initial versions. DDR3-2000 memory with 9-9-9-28 latency (9 ns) was available in time to coincide with the Intel Core i7 release. CAS latency of 9 at 1000 MHz (DDR3-2000) is 9 ns, while CAS latency of 7 at 667 MHz (DDR3-1333) is 10.5 ns.

Consider this equation:

CAS ÷ Frequency (MHz)) × 1000 = X ns

(7 ÷ 667) × 1000 = 10.4948 ns

 

[tecmo34]

Here is the definition of CAS/Latency... http://en.wikipedia.org/wiki/CAS_latency

Basically, the lower the CAS the faster the RAM will process data based the DDR level. It is independant of the MHz to a point. Examples, DDR3 is faster than DDR2, DDR3 1333 Cas6 is faster than DDR3 1600 Cas9... etc.

 

[Axonn]

Hello T_T, tecmo34, thank you for answering ::- ).

T_T: what I found was a 7-7-7-20 with a 1333 module. I suppose this is good ::- ). This ones:

http://www.webhallen.com/hardvara/113568-kingston_hyperx_genesis_4096mb_ddr3_pc3-10666_1333mhz

Do you recommend buying higher than 1333 modules? Intel says that Core i5 (for example) uses 1066 to 1333.

 

[tecmo34]

If you are using the Core i5, don't get that RAM!!! The voltage requirement for the CPU is 1.65V or less and that RAM is spec'd to run at 1.7V.

If you are running an AM3, it isn't an issue as they don't have the voltage restrictions like Intel.

 

[Axonn]

Awwww... ***... do I have to worry about voltages too??? I can't have RAM running at different voltage than CPU? Since when? For example I got a Core2Duo running at different voltage than the RAM right now.... *scratches head*.

 

[tecmo34]

With the newer Intel chips, they have a memory controller on the chip which puts a limit on the voltage requirements for this reason. It is something relatively newer requirement on the Core i3, i5, i7's.

 

[Axonn]

Awwwww, I see... Ugh...

So then what CPUs can use that RAM? The one I posted I mean... only AMDs? The website should have specified that, grrr. Luckily I didn't buy anything yet.

 

[tecmo34]

AMD or older LGA 775 Intel Socket motherboards with DDR3 support.

 

[abswindows7]

The ram frequency/cas formula is
1 / ((ram speed*cas)*2000)

1/2000*6*2000=6ns for a ddr3 2000mhz cas 6

1/667*7*2000=20.98ns for a ddr2 667mhz cas 7