ST STM32L4x6 Reference Manual page 1249

RM0351 Low-power universal asynchronous receiver transmitter (LPUART)
An Idle character is interpreted as an entire frame of "1"s. (The number of "1" 's will include
the number of stop bits).
A Break character is interpreted on receiving "0"s for a frame period. At the end of the
break frame, the transmitter inserts 2 stop bits.
Transmission and reception are driven by a common baud rate generator, the clock for each
is generated when the enable bit is set respectively for the transmitter and receiver.
The details of each block is given below.
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