Warning for those running low vcore but high vdimm on A64

swiftynz · Post by **swiftynz** » Sun Nov 13, 2005 8:29 pm

http://www.xtremesystems.org/forums/sho ... hp?t=79509

Saw this the other day, apparently by running a high vdimm and low vcore it is possible to kill your 90nm Athlon64.

Originally Posted by Tony
If you run a high vdimm and a low vcore with E die etc you are running the risk of killing the cpu's. So if you have VX etc and you are not raising the vcore on your cpu you stand a chance of pumping a lot of current into the memory controller and causing permenant damage.

Im seeing people running $1k cpu's at default vcore and BH5 at 3.4V, this is a sure way to kill the cpu and is NOTHING to do with an issue on the board. If you are going to run high vdimm run a higher vcore also, and keep your CPU's ok.

Tony works for OCZ, so you'd think he knows what he's talking about. From page2:

Originally Posted by Micutzu
First of all, the CPU doesn't have to do directly with the VDD, but with Vtt/Vref, wich should vary from 1.25 to 2V when you adjust the RAM from 2.5 to 4V. Maximum JEDEC Vdd voltage is 2.85V, and lowest voltage for A64 CPU's (CnQ activated) is 1.1V, so we know for sure that a difference of 0.325V between Vcore and Vtt is safe. This means that for a 1.35V Vcore voltage, we can have a safe Vdd of at least 3.35V, and for 1.5V Vcore -> 3.65V Vdd.
Probably the real danger is when those diodes go near the opening (conduction) voltage, wich should be ~0.6V for Si-based junction; if this is true, 1.35V Vcore and above 3.9V Vdd should kill the CPU instantly ... anyone wanna try ?

This has particular implications for silencers even at stock vdimm, because with undervolting the difference between the vcore and vtt can become quite large.

My 3000+ is currently set to run at 1.0V when idle, and my ram voltage is at 2.6v. Vtt (or Vref) is therefore 1.3v, giving a difference of 0.3v... So I'm safe, but without even realising it I was pretty close to the limit.

=assassin= · Post by **=assassin=** » Thu Nov 17, 2005 9:08 am

Sounds odd - if this problem exists it's a hidden bug not many people would expect. Could anyone else confirm or provide evidence of the authenticity of the statement? (I.e. is it true?)

=assassin= · Post by **=assassin=** » Thu Nov 17, 2005 9:13 am

Just wondering - if cool and quiet only affects the CPU Vcore - i.e. drops it down to 1.1v(?) then surely it can't be true, as what is essentially being done is the same? [EDIT] Helps If I read what was said - i.e. the 0.3 safe difference.

widman · Post by **widman** » Tue Nov 29, 2005 7:36 am

I doubt this theory, cause in reality I just out of limit.
I'm running CPU AMD64 4000+ @ 0.875V and VDimm at 2.6V (default). The different are 0.425V, more than 0.3V limit. It has been run for 6 months without any problem.

yeah, it's not 90nm die, but where is the exception?

Aris · Post by **Aris** » Tue Nov 29, 2005 8:10 am

even IF this is true, i doubt it will effect most of the people on this forum. he was talking about people overclocking their FSB and leaving cpu voltage at stock speeds. not people that leave FSB at stock and undervolt the cpu.

swiftynz · Post by **swiftynz** » Tue Nov 29, 2005 12:38 pm

Yea, it probably won't affect many people here because I don't imagine many would overvolt the ram. Still, there might be someone running the vcore at 0.9V or something, which would be a risk if this information is correct.

I don't have the knowledge to argue whether it is true or not, but in my eyes the source is credible enough.

darthan · Post by **darthan** » Tue Nov 29, 2005 2:22 pm

ok, to repeat what was said Micutzu is not talking about the difference between Vcore and Vdimm. Micutzu is talking about the difference between Vcore and Vtt/Vref. Vtt varies proportionally with Vdimm so the higher your Vdimm voltage is the higher your Vtt voltage is but 4V Vdimm does not equal 4V Vtt in fact, as he specifically states, 4V Vdimm equates to a 2V Vtt. If the difference between Vtt and Vcore is some (unspecified and unknown) amount greater than 0.325V then you can expect potential CPU death. He suggests that the

real danger is when those diodes go near the opening (conduction) voltage

and so the real roblems probably show up when the difference is ~0.6V. Therefore it would be unsafe to run your RAM at 4V (2V Vtt) and your CPU at stock 1.35V Vcore because that's a difference of 0.65V. It is not, necessarily, unsafe to run your Vcore at 0.875V and RAM at standard 2.5V (Vtt=1.25V) because that is a difference of just 0.375V and is probably within tolerances.

dragmor · Post by **dragmor** » Tue Nov 29, 2005 4:21 pm

I read this a couple of weeks ago and thought it might have impliciations but I've had no problems so far (3+ months) with my CPU at 0.8v and Mem at 2.8v. Could the temperature of the processor have an effect on this as well?

Aris · Post by **Aris** » Tue Nov 29, 2005 4:27 pm

dragmor wrote:I read this a couple of weeks ago and thought it might have impliciations but I've had no problems so far (3+ months) with my CPU at 0.8v and Mem at 2.8v. Could the temperature of the processor have an effect on this as well?

if your refering to the processor getting too cold. no, ive seen people use liquid nitrogen on their cpu's without any perminant damage.

and i would think if it got too hot it would auto throttle or just shut the pc down.