Check the interrupts section of the datasheet, it tells you how many clock cycles that takes - but with 250ns, you have only 4 clock cycles to do it in, which I don't think is enough, no matter how you do it.
Check the interrupts section of the datasheet, it tells you how many clock cycles that takes - but with 250ns, you have only 4 clock cycles to do it in, which I don't think is enough, no matter how you do it.