ddr5.md 1.6 KB
DDR5 Memory
Besides the insane RAM prices due to AI gobbling up the market, I made a few big mistakes that I wanted document.
No Quad Channel
First, I made a mistake with a RAM purchase back in 2024.
I thought quad-channel was still the superior choice, but the move to AM5 apparently carried some big changes to specifications.
Accordingly the maximum speed tops out at 3600 when running quad-channel, which is a significant performance hit.
I did find it strange that nobody was selling quad channel kits, but at the time AM5 was new and nobody had clearly explained this limitation and I did not RTFM...
I ordered two dual channel kits assuming that would work, and it did technically, but had a massive Code 15 at boot caused by setting EXPO 6000 when it could not operate above 3600.
This forced it to run extensive memory checks at each boot to find a stable speed prior to actually starting. Sometimes it would take over 10 minutes.
Mixed Mode Refresh
Apparently I encountered a second problem even after switching to dual channel memory.
When running memtest86+ using EXPO 6000 I encountered errors.
These errors disappeared if I ran in single channel, for either stick.
I tried:
- A different motherboard
- Three different memory kits
Each had basically the same errors.
Turns out there is a new default called mixed mode refresh that messes with the tRFC1, tRFC2, and tRFCSb values.
I managed to eliminate the instability by setting tRFC2 = tRFC1, then setting tRFCSb = tRFC2 / 1.625.
conclusions
It never hurts to read the manual.
DDR5 is expensive and volatile.