I'm aware the Intel i287XL is based on the later 387SX and that it both a process improvement and an architecture improvement over the original i287 floating point unit.

I'm also aware that the i287XL operates internally at 3/2 of whatever clock it is fed, unlike the earlier designs which are 2/3.

However, what is the actual safe operating range of frequencies for the processor? They don't appear to be speed marked or binned from what I can tell from the case markings, whereas the original Intel, AMD (and probably others, too) 287 definitely is:

The reason being is that now with access to several 286 boards, I find that they all seem to have a different strategy for feeding a clock signal to the FPU: one of them has no seperate oscillator for the FPU and (I assume) feeds it some fraction of the processor clock. Another board has an option for running the FPU at one fo two fixed divisors of the processor, a third has an option to run the FPU at a constant 14.318MHz or feed it via another (optional) oscillator frequency of your choice (either an integer divisor of the main clock, and run it synchronously, or a non-divisible frequency and run it asynchronously).

It seems that there was no standard approach to clocking the FPU and therefore the possible frequencies that these things could run at could vary wildly.

What, therefore, is the safe (internal) operating frequency of the i287XL to run at?