A bigger issue in timing accuracy may well be the latency of your interrupt handler. Nigel has already noted that the time between the start bit and the UART's determination of a FE will be failry consistent. Of course you also need to consider the "data" the UART acquired at the time of the FE. If it is not 0x00, it may well be a FE, but is most certainly not the beginning of a BREAK.
Peter
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