9800XT Vgpu mod

[bmg]

I started a thread on the 9800XT Vdd mod: http://www.xtremesystems.org/forums/...threadid=20855.

I figured I'd start another thread for the GPU vmod. The PWM controller on the 9800XT is different from the 9800pro, it's the Fairchild FAN5240MTC, http://www.fairchildsemi.com/pf/FA/FAN5240.html. I've just started studying the data sheet, but the interesting thing about this part is that it's original intended application was as a two phase PWM controller for the AMD mobile Athlon and Duron processors. Voltage is supposed to be adjustable from .0.925V to 2.00V, with a 5 bit input for voltage selection. If we're lucky it might be possible to control these directly, although some may be tied directly to ground, which would complicate things.

Anyway, I thought I'd pass on this info so we can get started working out a mod. Hopefully I'll have some time this weekend to work on it.

[Kunaak]

keep up the good work, I am sure alot of people are waiting for the results

[bmg]

Here's more information on the 9800XT Vgpu vmod. As mentioned above, the 5240 controller has 5 voltage select pins, VID4-VID0. The 9800XT has them hardwired: VID0,VID1 and VID4 are grounded and VID2 and VID3 are high. This normally gives an output voltage of 1.40V. However, unlike the example circuit in the data sheet, the 9800XT has a resistor between pin 17 and pin18 (VcoreD and Vcore+), as well as the series resistor between pin 18 and the Vgpu voltage. The resistor between pins 17 and 18 results in a voltage drop from Vgpu to pin 18. On the 9800XT the resistor between pins 17 and 18 is R1591 and the series resistor is R1592. The Vgpu voltage is thus:

Vgpu=1.40x(1+R1592/R1591)

The original resistor values are:
R1592= 432 ohms (62A code)
R1591=1620 ohms (21B code)

This give a default Vgpu=1.4x(1+432/1620)=1.77V. The measured value is 1.78V, which is excellent agreement. To increase Vgpu the easiest approach is to reduce the resistance between pins 17 and 18 by carefully connecting a pot across R1591. I'd suggest using a 20K pot. When the pot is at 20K, Vgpu would be 1.80V. The pot would be at 7.9K for 1.85V and 4.8K for 1.90V. If you want to go above 1.90V, a 10K pot might be a better choice.

Here's a picture showing the connections for the pot as well as convenient points for measure Vgpu. (Sorry for the crappy picture quality.)

[charlie]

Great Info, BMG!!!
Thanks. I'm sure someone with a SUPER digi cam will come along and do a close-up of the resistors in question and then we will ALL be brave enough to try the mod!!
C

[Bra!nFreeZe^]

Nice ! but if it was me that just got a 9800 XT i wouldnt have the nerves to vmod it !

[Riska]

Hi Bmg is it possible to just change the SMD resistor like you said in the vmem mod??
( I'll most likely just replace the 562 ohm resistor on the board with a 487 ohm (for 2.98V) and then remove the wires.)


Please help me if you can!

[bmg]

Originally posted by Riska
Hi Bmg is it possible to just change the SMD resistor like you said in the vmem mod??
( I'll most likely just replace the 562 ohm resistor on the board with a 487 ohm (for 2.98V) and then remove the wires.)

Please help me if you can!

YGPM.

[Riska]

I have Pmed you back!!

[Riska]

Bmg have you read my PM?

[OPPAINTER]

I can't take a good picture but here's mine for those PMing me.

OPP

[Charles Wirth]

I have had my GPU up to 2.1v
Memory up to 2.9v

Card still works fine.

What volts you guys run up to?

[xDUCK]

Originally posted by FUGGER
I have had my GPU up to 2.1v
Memory up to 2.9v

Card still works fine.

What volts you guys run up to?

Is that a typo, or are you just really conservative?

[charlie]

Maybe he meant 3.9V, lol

C

[bmg]

Originally posted by charlie
Maybe he meant 3.9V, lol

C

The memory spec is 2.80+/-.15V, but the default voltage is around 2.68V. I've found cases with my two 9800XT's where increasing much above 2.8V actually increased artifacts. This was gaming not necessarily benching. While running Unreal2 my first 9800XT had fewer artifacts (sparkle) as mem voltage was increased. I ended up running it at 3.0V. However, when I was playing Halo I had sparkle artifacts in some places, and reducing voltage below 3.0V reduced the artifacts. I ended up running at 2.85V as a compromise. My second 9800XT has much better performing memory than my first card, but I don't see a lot of memory improvement as memory Vdd voltage is increased. I'm wondering if Vddq or Vtt is limiting my second card. The core definitely isn't the problem.

[charlie]

BMG,
My current XT, the one that is a lousy performer will give a hard lock at everything above 466....how does that compare to your XT's out of the box?

C

[Charles Wirth]

Not a Typo, thats all I need to max the memory out at ~451Mhz +-5Mhz

My current GPU block running R410

[charlie]

Fugger, have you tried out the new toy yet??? Awfully pretty!!

C

[Charles Wirth]

yep

[xxORBxx]

wow thats one shiny umm....evap head...


is there a VDDQ that might be holding the mem back? I know the VDDQ is a problem on my 9800p mem, stuck at 378

[runmc]

hey opp,
It looks as though you've used a different resistor than bmg???

Did you solder to the ass end of the fat tan resistor on the end closet to the controller??

[Boogotop]

where the voltage measure point for the gpu???

[bmg]

Originally posted by Boogotop
where the voltage measure point for the gpu???

Check out the picture with "Vg" written on a piece of tape. At the tip of that tape there's a feedthrough without any solder mask on it. That's connected to Vgpu.

[Boogotop]

Originally posted by bmg
Check out the picture with "Vg" written on a piece of tape. At the tip of that tape there's a feedthrough without any solder mask on it. That's connected to Vgpu.

thx my friend!

[toolbox]

Does someone have a picture close up of the whole thing. Without tape covering the resistor?

[daos]

so basically, with a 20k pot, its impossible to adjust the voltage to the vGpu voltage any higher than 1.9v? reason i ask is because im a n00b to this and was worried about over adjusting the pot and frying my chip.

if i cannot go any higher than 1.9v using a 20k pot then i would consider that the safest route. any comments?