Proclaimed CAS vs actual CAS

[EvilCabal]

Hi,

I just bought 2x1gig dual channel OCZ Platinum DDR2 PC-6400 ram (OCZP800R21G). OCZ states that CAS is 4,4,4,15 (

http://www.ocztechnology.com/products/memory/ocz_ddr2_pc2_6400_platinum_revision_2_dual_channel)

However, CPU-Z says it is actually 5,5,5,15 at 400Mhz. It is 4,5,5,13 at 333 and 4,4,4,10 at 270. What does this means? Is the announced CAS a downclocked CAS to appear better? Is CPU-Z wrong? Or is it something else?

Thanks

Fred

 

[cdogg]

I'm thinking it's something else. PC-6400 is optimized for 800MHz. So that means the memory controller has to be clocked at 400MHz (external) in order for the memory to be running at 800MHz (internal), hence the meaning behind DDR.

I think what you are seeing in CPU-Z is the "internal" clock frequency. So the 400MHz rating is assuming that the memory is running on a 200MHz bus.

To know for sure, we'd have to have more info about your motherboard and CPU such as the make/model. There's a big difference in the way speed is calculated between AMD and Intel's newer chipsets.

~cdogg
"Insanity: doing the same thing over and over again and expecting different results." - Albert Einstein
[tab][navy]For general rules and guidelines to get better answers, click here:[/navy] faq219-2884

 

[EvilCabal]

No no, I know what you mean but my FSB is at 800 mhz, CPU-Z shows the ram 400 mhz clock. But since it's dual channel it's 2x400 = 800MHz. My board is P5W DH Deluxe. My CPU is an intel p4 3.2GHz. Any clues?

Thanks.

 

[paparazi]

The clues are:
Yor motherboard appears not to be automatically recognising the memories capabilities so is setting default values.
This is pretty normal behaviour for performance memory types and means that you will need to set these values: voltage and timing settings etc manually in the bios.
The reason for this is: that in order for these faster timings to work, generally the memory needs slightly more voltage.
This is not overclocking as the memory is designed to run at these voltages and timings as standard.

I have the same motherboard as you and had to raise dimm voltage to get stable and reliable results from my Giel
PC-6400 DDR800 running at faster timings.
Martin



We like members to GIVE and not just TAKE.
Participate and help others.

 

[EvilCabal]

Yea... I thought it could be that. In fact I had to manually set the ram at 800 mhz because it was running at 667mhz when set to [Auto]. Do I need to adjust the voltage or do changing the timings will automatically adjust the it. If not, how do you figure what voltage is ok? Is this part of the ram spec?

Thanks again.

 

[kmcferrin]

No no, I know what you mean but my FSB is at 800 mhz, CPU-Z shows the ram 400 mhz clock. But since it's dual channel it's 2x400 = 800MHz

That's not what dual channel is. Dual channel doesn't have anything to do with clock speed, it's all about bit width. With a single channel memory controller, you would have a single 64-bit memory path (channel) between RAM and CPU. With dual channel controllers, you have a pair of 64-bit memory paths (channels) between RAM and CPU that function effectively as a since 128-bit wide path. This alleviates some of the memory/CPU bottleneck.

DDR2-800 memory really runs internally at 200 MHz. But since it is DDR2, the memory bus runs at twice the speed of the cells, at 400 MHz. Then because of it being double data rate, it transmits data on the rising and falling edge of the clock signal, which results in an 800speed rating (i.e., DDR2-800 runs at 400 MHz, DDR-400 runs at 200 MHz, etc).

 

[cdogg]

EvilCabal,

You're FSB's true speed is 200MHz, not 800MHz. Intel P4 "quad-pumps" that speed by sending four times as much data per clock cycle. That's why it's "rated" at 800MHz without really running that fast.

Your memory bus is also running at a ratio of the FSB. For the best efficiency, this ratio should be 1:1 which would also put the memory bus at 200MHz. DDR doubles that to 400MHz, and here's where you're getting confused. Each memory stick is running at 400MHz. Together in dual-channel, they put out the same bandwidth that a single 800MHz stick can, but each runs at 400MHz separately.

In your situation, it would have been better to use two lower latency PC2-3200 sticks, which run at 400MHz. PC2-6400 is for overclocking and for the future Core processors from Intel that will have a 1600MHz FSB.

~cdogg
"Insanity: doing the same thing over and over again and expecting different results." - Albert Einstein
[tab][navy]For general rules and guidelines to get better answers, click here:[/navy] faq219-2884

 

[kmcferrin]

I still think that the case is being mis-stated regarding dual channel and memory bandwidth. What you're getting with dual channel is a wider path, not a faster path. If you have a 64-lane highway with a 60 MPH speed limit, you can only move 64 cars at a time. If you have 128 cars to move, then half of them will arrive after the other half. If you expand the highway to 128 lanes, then you can now move twice as many cars in half the time. But the highway still has it's 60 MPH speed limit. If you only have 64 cars to move, it's still going to take you the same amount of time regardless of whether you have 64 or 128 lanes of freeway. You can move more cars in the same amount of time, but you can't move the same number of cars there any faster by going to dual channel.

Sorry to mix metaphors.

I think where people are getting confused is in thinking that the "2" in DDR2 has anything to do with the number of modules, so lets backtrack.

In the beginning there was SDRAM. SDRAM debuted at around 66 MHz and peaked at around 133 MHz. In both cases the memory clock and the I/O bus clock were the same.

Then DDR SDRAM came along. DDR SDRAM started at around 133 MHz, but since DDR transmits data on the rising and falling edge of the clock signal it could send twice as much data at the same clock speed. Therefore the first DDR modules running at 133 MHz were considered 266 MHz effective. Sometimes they were referred to as 133/266. Then a little later we had 166/333, then 200/400. There were even some modules that were produced at even faster speeds (DDR433 and DDR500, running at 217 and 250 MHz respectively) that were popular with overclockers. In all cases the memory clock and the I/O bus clock were the same.

Then came along DDR2 SDRAM. DDR2 took the previous improvements, and added to them another trick. DDR2 runs the I/O bus at twice the speed of the memory cells, whereas before the bus and cells were at a 1:1 ratio, they are at 2:1 in DDR2. With regular DDR, the cells that ran at 200 MHz memory clock were run on a 200 MHz I/O bus clock, but they had an effective 400 MHz speed because they transmitted on the rising and falling edges of the signal. Now with DDR2 those cells still run at 200 MHz memory clock but with a 400 MHz I/O bus clock. This results in twice the data being transferred per cycle (versus DDR SDRAM), and in the case of the 200 MHz memory cells an effective 800 MHz speed.

DDR2 has nothing to do with the number of modules, nor does it have anything to do with single/dual channel modes. It is absolutely possible to run 800 MHz DDR2 memory at 800 MHz in single-channel mode.

So to return to your comment above:

Each memory stick is running at 400MHz. Together in dual-channel, they put out the same bandwidth that a single 800MHz stick can, but each runs at 400MHz separately.

This is not the case. Each module does run at 400 MHz separately. However, each module independently has the same amount of bandwidth as a theoretical "single 800 MHz stick" and is essentially a single 800 MHz stick. However, each stick independently can use only a 64-bit wide bus. But together in dual channel mode they have a combined 128-bit wide bus to use. Bus width has nothing to do with the bus clock.

 

[paparazi]

It appears your Vdimm voltage range is between 1.9 and 2.1volts on this particular OCZ ram, I think ram slot default is 1.8volts (but check) that means a manually upping at least 0.1volts (I took mine up to 2.0volts and it has proved solid and reliable)
You need to read some reviews for this ram but usually the stated timings and operating voltage are supplied somewhere on the packaging.
Martin

We like members to GIVE and not just TAKE.
Participate and help others.

 

[cdogg]

kmcferrin,
Thanks for the correction. If you were to re-read my last post again inserting DDR-400 (PC3200) in place of DDR2, then it would make sense. Somehow I think I mixed myself up forgetting that DDR2 is essentially "quad-pumping" the FSB using the technology that you elegantly described. That was my bad.

As you can see in the following thread, I got it mostly right last time I spoke on the subject:
thread602-1310526

Not sure how I forgot about that, but maybe it is a sign that it's time for a vacation!

Your analogy of expanding the number of lanes on a highway without changing the speed limit is a good one to use too. I know I've used that several times over the years!


_________________

Fred,
I think what kmcferrin and I have failed to do throughout this mixup is explain why your timings aren't right. Out of curiosity, does CPU-Z report anything different when you only have one stick installed? I'm wondering if it's having trouble getting an accurate reading while they're running in dual-channel? Make absolutely certain that the FSB is running at 200MHz. Every increment or 1MHz faster will have an affect on the memory timings.

~cdogg
"Insanity: doing the same thing over and over again and expecting different results." - Albert Einstein
[tab][navy]For general rules and guidelines to get better answers, click here:[/navy] faq219-2884

 

[EvilCabal]

Allright, thanks to you for the answers. I will try this soon but now my motherboard or cpu or psu has broken and my pc won't boot anymore...

Btw cdogg, I am pretty sure that PC-6400 is intended for 800Mhz FSB and not 1600.

 

[kmcferrin]

Yeah, I did forget that, didn't I? I think that Paparazi has it right.

The official DDR2 spec is either 1.8 or 1.9 volts (I don't recall which offhand). Because of the techniques used to run the I/O bus at twice the memory bus speed, you get much higher latencies with DDR2 than with DDR. But as overclockers discovered long ago, if you up the voltage you can run more aggressive memory timings. So most DDR2 memory modules today are specified by the manufacturer to use a higher voltage than the official spec in order to get lower latencies (some going as high as 2.1 or 2.2 volts).

I suspect that if you up the memory voltage to whatever OCZ has specified then your modules can be manually set to run at 4-4-4-15. This shouldn't be a problem since you are still running them within OCZ's specs.

The only time that there might be a problem is if your motherboard doesn't support memory voltage adjustments. I have seen some boards with Intel-based chipsets that specifically state that they only run DDR2 at the official spec voltage. In that case you would be stuck running at the 5-5-5-15 timings.

 

[cdogg]

Yes, the voltage may well be the problem here.

Fred,
We were both actually right. PC2-6400 can run in single-channel mode at 800MHz which matches the old P4 800MHz FSB (again, kmcferrin provided a lot of detail about that above). Matching is a good thing. So dual-channel is not necessary in your situation.

If you do run PC2-6400 in dual-channel, then you get the equivalent "bandwidth" of 1600MHz, which would be perfect for a 1600MHz FSB. You can still run it in dual-channel for your old P4 800MHz FSB, but it won't provide much benefit over single-channel.



~cdogg
"Insanity: doing the same thing over and over again and expecting different results." - Albert Einstein
[tab][navy]For general rules and guidelines to get better answers, click here:[/navy] faq219-2884

 

[EvilCabal]

Maybe I misunderstand you but I am confident that dual channel pc-6400 was meant for 800Mhz FSB. The internal ram speed being 400Mhz. But since it transfers data on both the rising and falling clock edge, it is rated as 800.

The fact that it is dual channel only gives you a wider path, as it is cleary explained a few times this thread.

As you can see it here:

http://www.tomshardware.com/2007/06/15/hardcore_ddr2_ram_by_corsair/

Dual channel PC 6400 runs at 400Mhz, anything above it is considered overclocking.

 

[cdogg]

Yeah, all the terminology can get quite confusing.

When the Athlon XP Barton processors came out years ago that supported the 400MHz FSB, we first saw dual-channel being included on some boards. In fact, I had one! PC3200 DDR already ran at 400MHz matching the Athlon's FSB, so the big question was whether or not dual-channel really made a difference or was needed at all. In the end, most benchmarks only showed about a 5-10% improvement on average when dual-channel was enabled.

Later, the wider path finally became a factor and helped us match the bandwidth of faster FSB's. For example using dual-channel, DDR 266 matched the 533MHz FSB and DDR 400 matched the 800MHz FSB.


DDR 400 = PC3200 = 3.2 GB/s
Dual-channel PC3200 = 6.4 GB/s
800 MHz FSB = 6.4 GB/s

DDR2 800 = PC2-6400 = 6.4 GB/s
Dual-channel PC2-6400 = 12.8 GB/s
1600 MHz FSB = 12.8 GB/s

You can see why they use the model numbers like PC3200 which refer to the bandwidth. My point before is that without dual-channel, one stick of PC2-6400 already has the same bandwidth as the 800MHz FSB. With dual-channel, PC2-6400 equals the same bandwidth as a 1600MHz FSB which is 12.8 GB/s.

Maybe it's more clear now when we focus on bandwidth instead of speed!

~cdogg
"Insanity: doing the same thing over and over again and expecting different results." - Albert Einstein
[tab][navy]For general rules and guidelines to get better answers, click here:[/navy] faq219-2884