Visa info for non-EU citizens on extended stays

I am American, looking to do personal travel in Europe for the next 1 to 1.5 years. I am not seeking employment, or work, and plan to use Berlin
as my base. I will be returning to the U.S. every 6-8 weeks.
I am trying to determine if there is any registration requirement or other requirements needed in Germany. I also realize that there is an automatic 90 day visa granted to US citizens, but I have also seen references to 90 days within 6 months that I am a bit unclear on. Does that mean that as an american, I can only be in German/EU/Schengen countries for a total of 90 days within a 6 months period. Does the fact that I return to the US every 6-8 weeks, for 1-2 weeks mean that that clock is restarted?

Thanks in advance.

i'm also very curious about this! i have my US passport which allows for 90 days with no visa.
my tourist stay is only for another 2 weeks, and i thought maybe i could just leave the EU for a day, get my passport stamped, and have another 90 days free and clear...but then someone told me the other day that if i left without having a visa, that i couldn't return for another 6 months.

then, today somebody else told me that is bullshit about the 6 month thing...i'm confused and don't know who to believe!

Have a look US passport renewal