Hi,
Ok, setting SMPS6 to 1.5V does not change anything at 1.5Ghz. So like I described there is another problem behind this onion shell. Maybe this time it is really an hardware (capacitor) issue.
I had some thoughts about my results, maybe here are some ideas that might put you onto the right track:
Based on my experience so far, the higher clockspeed a CPU runs, the higher the voltage it needs has to be. To overclock a CPU people often overvolt it.
-> It's not only the overall power supply that's important here, but also the voltage. 5W is in this case not 5W.
Also based on my experience is the behaviour of a CPU when you overclock it: The further you overclock, the more calculation errors it does. That's why it becomes unstable the more you overclock it. That's also showing in my tests: While my board needs about 20 - 30 seconds to freeze running with 1,19GHz and 1,06V, it already shows lots of kernel errors until then, which means the processor goes haywire.
Increasing the speed to 1,2GHz makes it freeze right away.
-> The CPU behaves exactly like it's overclocking (which is is at 1,06V)
Increasing the voltage to 1,25V improves things a bit (it takes 10 - 15 seconds more until it crashes), however, at 1,25V, the OMAP5 should work fine.
-> It looks like the voltage is not high enough
Interesting though is that it works better if I add more inbetween frequencies with inbetween voltages which lets me run it stable with 0,06 GHz more.
-> Besides the voltage, we seem to also have the effect that something goes wrong with sudden voltage changes
The PALMAS should take care of the voltage, and it does so by measuring the voltage at the OMAP5, to compensate any trace resistance values we have.
That doesn't seem to work for higher voltages (1,25V) though.
-> Something is lowering the voltage OR something causes the voltage to be unstable (as the OMAP5 is a bit picky and the voltage should be mostly stable). Our traces have been measured and seem to be okay, so that doesn't seem to be the issue.
Now combining that with what I learned about capacitors (I'm not sure I got everything correct...)
Capacitors also have a resistance which changes based on the frequency (ESL). This is what TI also notes in their documents. Also, capacity changes with the voltage, which could probably be a factor in our case as well.
If I understood correctly, as different capacitors behave differently with these parameters, you use multiple different sized ones in such cases. This should minimize the capacity loss and also the resistance leading to a much more stable voltage. As current usually takes the way of the least resistance, multiple capacitors should improve things here, right?
Another thing that comes to my mind: If the capacity / resistance changes in a capacitor, could it be the PALMAS is constantly changing the voltage when running at 1,25V to compensate that? So basically that the changing resistance / capacity of the capacitor is causing the PALMAS to always trying to correct the voltage?
In that case, we'd have a pretty unstable voltage, which could also explain why it works better in case we don't have a sudden increase from 1,06V to 1,25V but using many voltage steps inbetween.
Another idea I had: AFAIK, the bigger a capacitor is, the longer it takes to adapt to a new voltage. So maybe our capacitor is too big which means that the CPU clocks higher before it's getting the higher stable voltage. That would also explain why voltage steps improve things.
The question for me is:
* What role plays the total capacity (with all capacitors combined)? * What role plays the position of the capacitors? If it's more about having different valued capacitors instead of just one, then manually soldering them in parallel to the existing ones we have on the CPU board might improve the situation? * What role play the single capacity of a capacitor? Do larger ones have a larger increase of resistance / decrease of capacity with higher voltage than smaller ones? * What other differences are there when using two smaller capacitors compared to using one bigger one?
I guess something that's important is to simply try to replace existing capacitors with different ones or piggyback / add additional ones to the side or top.
It might not improve things, but it should at least have SOME effect (even if it makes things worse), right?