Well, your answer doesn't really solve my confusion.

If the internet sources are right, then a Tx should have a x MHz XTAL (externally) and the CPU should get x/2 MHz.
But if this is true, then all MHz values in your driver are wrong, because you're using now (x*2)_MHz_XTAL/2 and that's of course x MHz for the CPU.

So what's in fact correct now?