Ok guys. Just a quick post here...
so I recently bought a 5950X for use on my B450 MSI Tomahawk motherboard. I figured everything should work ok. However, I've had multiple problems.
First problem was that the motherboard's default PBO setting (or "auto") seems to cause the machine to crash. I don't really know why since it should BE OFF BY DEFAULT but whatever. If you set the setting to disabled or configure PBO to use the motherboard defined power limits you'll be good to go.I don't really understand why this is. The motherboard is capable of supplying more power than the CPU needs but if you turn it on, or just leave it as "auto", the machine is unstable. Using motherboard power limits (which are very high) seems to work fine though. WTF?
The second more surprising problem is that enabling XMP with my super low latency memory causes random (if somewhat infrequent) crashes. What happens is the screen goes blank and the system won't respond to any attempts to reset. You have to turn off the power supply, wait a few seconds, then turn it back on. It's a nasty crash. This is also the symptom if PBO is set to auto, btw. Initially I thought this was power related since I was using a cheap 550W power supply. But upgrading to a 750W good power supply did nothing.
In any case the MSI Tomahawk website's supported memory list gives a clue as to what is happening here. The motherboard supports different memory depending on the generation. The Ryzen 5000 series only supports memory up to 2666Mhz speeds.. Which is weird.. but is a big clue that there's some issue with the motherbaord, the 5000 series and high speed memory.
In any case I'm now running my memory at 2666Mhz but with crazy low timings of 12-12-12-28 thanks to having 3200Mhz memory with 14-14-14-34 timings running at a lower bus speed. Memory timings are specified in clock cycles but the timings don't change with clock speed so the numbers can be brought down when the bus speed decreases. So far it's stable and it's been a while.
I'm putting this out there because it took a stupidly long amount of time to figure this out so hopefully it will show up if someone searches for it. The forums have some of this info but the forums also have a bad signal to noise ratio.