I've created designs exclusively with 5 MHz xtal (16xPLL==80 MHz clock). It works. Well.
I now have speed pressures that would be helped by a faster clock. I've read "the documentation" and it seems to indicate that a 128 MHz clock is within spec. From the Propeller Manual, on the CLK register:
The Clock PLL's internal frequency must be kept within 64 MHz to 128 MHz this translates to an XIN frequency range of 4 MHz to 8 MHz.
I realize that challenges increase with higher clock frequency: power consumption, IR drop, noise, reliability. Is there a generally accepted maximum reliable
clock frequency that has been proven reliable (within defined constraints - including specific circuit design parameters (decoupling capacitors, wire length, etc) over the years?
Summarizing: it seems from the "official" documentation that an 8 MHz xtal and 16xPLL configuration is "legal". The question is whether it can be used reliably, and if so, what if any accommodations must be made?