Actually, you can just design your protocol so that the first byte sent to
the PIC can be ignored. Make that first byte X'00'.

Now:

Turn off UART
Sleep
Wake up on WDT

When you wake up, take a quick look at the UART input pin using a simple
BTFSx instruction.

If the pin is a one then:
    Go back to sleep.
Otherwise:
    Wait for the pin to become a zero (stop bit condition).
    Enable the UART.
    Receive the message.
    Process it.
    Go back to the top.
End if.

This scheme should work well if the character rate is signicantly slower
than the WDT interval.

Bob Ammerman
RAm Systems

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