Strange RAM timing values in CPU-Z: Has the BIOS selected wrong timings?

Hi,

I have an ASUS P5B-E Plus motherboard, equipped with 2x1GB PC4200 RAM (Corsair Value Select) and 2x2GB PC6400 RAM (Corsair XMS2).

In CPU-Z the following information is shown in the SPD tab:

* For the PC4200 RAMs:

JEDEC # 1 3.0-3-3-9-11 @ 200 MHz

JEDEC # 2 4.0-4-4-12-15 @ 266 MHz

* For the PC6400 RAMs:

JEDEC # 1 4.0-4-4-13-15 @ 270 MHz

JEDEC # 2 5.0-5-5-18-22 @ 400 MHz

As I understand it the BIOS should select the highest required timing values gathered from all the installed RAMs, at the highest supported common clock frequency. In this case that should be 4.0-4-4-13-15 @ 266 MHz.

But in CPU-Z the following information is shown in the Memory tab:

Memory Frequency 266.7 MHz (1:1)

CAS # latency (CL) 4.0

RAS # to CAS # delay (tRCD) 4

RAS # Precharge (tRP) 4

Cycle Time (tRAS) 12

Command Rate (CR) 2T

In other words, a tRAS value of 12 was selected instead of 13 (despite the fact that the PC6400 RAMs require a tRAS of 13 @ 266 MHz (or actually @ 270 MHz)).

Is there anyone who can explain why this happens? Has the BIOS selected the wrong timings for the RAMs, or could something else be wrong?

Also, how come the PC6400 RAMs have a JEDEC entry for 270 MHz and not 266 MHz? I thought the "standard frequencies" were 200, 266, 333, and 400 MHz?

Thanks in advance!

Thanks for your answers. I have some follow up questions:

A: The memory Controller will select the first set of modules detected and use the timings to post the system in most systems it will not post because of Miss matched SPD's

Does this mean that the Corsair Value Select PC4200 RAMs should not be considered compatible with the Corsair XMS2 PC6400 RAMs (due to the mismatched SPDs / JEDEC entries)?

A: The SPD values are set to JEDEC Standards for the specific speed grade they are set to, not what some chipset's support.

I'm sorry but I don't think I fully understood that, could you please clarify? With a "speed grade" do you mean e.g. PC4200, PC6400 and so on? If so, does it mean that (according to the JEDEC Standards) all PC6400 RAMs will have a 270 MHz JEDEC entry (but no 266 MHz entry), and all PC4200 RAMS will have a 266 MHz JEDEC entry (but of course no 270 MHz entry)?

If that is the case, and you say that the BIOS often fails to POST when there are mismatched JEDEC entries, does this mean that no PC4200 RAMs are considered compatible with any PC6400 RAMs?

Mixing memory has never been supported and or suggested so no its never a good idea to mix memory modules on any system. Now that does not mean it cannot be done, because as you found you can with your MB and some MB BIOS are better than other is about all I can say but it is a crap shoot and you just got lucky.

Well yes, the system boots up ok. However, since the BIOS has selected the RAM timings 4-4-4-12-15 @ 266 MHz and the PC6400 RAMs require 4-4-4-13-15 @ 270 MHz, doesn't that mean there's a risk that the system might be unstable?

Also, if one selects RAMs with the same speed (e.g. only PC4200 or only PC6400), isn't it ok to mix RAMs (as long as you used matched pairs in each RAM bank)? Not even if they have the exact same timings at the same frequency? If so, how come that is the case? :confused:

Thanks! I'm still curious as to why that is the case though. I mean, if a RAM module is clocked at a frequency it supports and the timings it requires are used, then how can it be affected by another RAM module (that supports and uses the same frequency and timings) placed in another RAM bank?

Also, I didn't get any answer to the question whether my system might be unstable due to the fact that the BIOS has selected non-supported timings for the PC6400 RAMs (see my previous post for details)?

Thanks in advance!

Now in your case more than likely the MB is just picking the first SPD range it see's IE Cas 4-4-4-12 at 266 MHz (The TRAS will have no effect on system performance so a setting of 12 or 13 is no difference to the system).

Thanks, good to know it won't affect the system performance! Does it also mean that setting a tRAS of 12 will have no affect on system stability either, even though the PC6400 RAMs require a tRAS of 13 @ 266 MHz?

Thanks! The BIOS guide was excellent. It will be useful for better understanding lots of other BIOS settings.

Anyway, the BIOS guide section closest matching what I was looking for was "SDRAM Tras Timing Value". It says:

"If the tRAS period is too long, it can reduce performance by unnecessarily delaying the deactivation of active rows. Reducing the tRAS period allows the active row to be deactivated earlier.

However, if the tRAS period is too short, there may not be enough time to complete a burst transfer. This reduces performance and data may be lost or corrupted."

To me that sounds like if the BIOS sets the tRAS too low (e.g. 12 instead of the required 13 in my case), this might cause system instability problems?

No normally the TRAS would or should be set to Cas + TRCD + TRP IE 4+4+4= 12 and 12 to 15 would be fine for that system at those cas settings.

Thanks, that sounds reasonable. But then, isn't it strange that the RAM's JEDEC entry reports that it requires the timings 4-4-4-13? Why does the RAM report that it requires a higher tRAS than it actually needs?