# Why does the $\rm H$-burning Shell do the $\rm CNO$ cycle instead of the $pp$ Chain?

A star that initially produces energy through the $$pp$$ Chain will eventually exhaust its Hydrogen leading to a core contraction in order to increase the temperature to generate again nuclear reactions. Since the $$pp$$ Chain has a temperature threshold smaller than the $$\rm CNO$$ cycle: Why is then the H-burning shell a $$\rm CNO$$-cycle one and not a $$pp$$-Chain one?

If the answer is: Because the temperature corresponds to the $$\rm CNO$$ cycle and not to the $$pp$$ Chain; then why the contraction does not halt when the temperature surrounding the Helium core is enough to fuse Hydrogen through the $$pp$$ Chain?