> Not all characters end up having either a zero or one that
> is only a single bit width wide.  IIRC, a string of capital "U"
> characters is nearly a perfect square wave

That's how the PIC EUARTs auto-baud

"Once the ABDEN bit is set, the state machine will clear
the BRG and look for a START bit. The Auto Baud
Detect must receive a byte with the value 55h (ASCII
"U", which is also the LIN bus Sync character), in order
to calculate the proper bit rate. The measurement is
taken over both a low and a high bit time in order to
minimize any effects caused by asymmetry of the
incoming signal. After a START bit, the SPBRG begins
counting up using the preselected clock source on the
first rising edge of RX. After eight bits on the RX pin, or
the fifth rising edge, an accumulated value totalling the
proper BRG period is left in the SPBRGH:SPBRG registers.
Once the 5th edge is seen (should correspond to
the STOP bit), the ABDEN bit is automatically cleared"

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