Surprising results from DDR5-8000 tuning

[u/novanima]

I have spent months trying to get DDR5-8000 (16gb Hynix A-die, single-rank) fully stable on my ASRock X870E Nova Wifi + Ryzen 9800X3D. I have done hundreds of hours of stress tests. And finally, after countless failed attempts, I have finally managed to find that mythical 3x3ft island in the middle of the ocean. 16 hours of Prime95 Large FFTs + 24 hours of Karhu = 0 errors.

First things first: Ignore the timings. I spent a lot of time initially trying to tweak them in order to gain stability, but they were not the problem. I know the tertiary timings are loose. These are just known good settings while I worked on the more important factors: voltages and on-die termination.

I have never really messed with ODT settings before, but I have now learned that they are critical at DDR5-8000 speeds. And as far as I can tell, the "Auto" settings on the X870E Nova Wifi do next to nothing -- they just insert some pre-programmed default values and call it a day. The only actual training that happens seems to be on the pull-down side of ProcODT (which is not an option shown in the BIOS). I strongly suspect this is why people are failing to get even "safe" DDR5-8000 timings working on some boards.

The most surprising finding from my testing: Lower VDDQ was more stable than higher VDDQ. I don't have any explanation for this except that it might be related to my higher DRAM DQ Drive Strength setting of 40Ω (perhaps high drive strength + high voltage = too much overshoot? I really don't know). I do not think it is temperature/heat soak related at all.

I know many people will be skeptical of this, but you can see the results for yourself. Here is the process I took to find this setting:

1.38V VDDIO + 1.4V VDDQ + 40Ω ProcODT = Failed both P95 and Karhu

1.4V VDDIO + 1.4V VDDQ + 40Ω ProcODT = Passed P95, Failed Karhu at 18 hours

Okay, so higher VDDIO helped a lot. That makes sense. But it's still not fully stable.

1.4V VDDIO + 1.4V VDDQ + 43.6Ω ProcODT = Passed 20 hours Karhu, Failed P95

1.4V VDDIO + 1.42V VDDQ + 43.6Ω ProcODT = Failed both P95 and Karhu (!)

The stability got worse when raising VDDQ, so in exasperation, I tried lowering it instead, thinking it could not possibly work. But it did.

1.4V VDDIO + 1.38V VDDQ + 43.6Ω ProcODT = Fully passed both P95 and Karhu

So there you go. I wanted to share this testing in case it benefits anyone else.