ST STM32L4+ Series Reference Manual page 1706

Inter-integrated circuit (I2C) interface
Bit 11 ADD10: 10-bit addressing mode (master mode)
Note: Changing this bit when the START bit is set is not allowed.
Bit 10 RD_WRN: Transfer direction (master mode)
Note: Changing this bit when the START bit is set is not allowed.
Bits 9:0 SADD[9:0]: Slave address (master mode)
Note: Changing these bits when the START bit is set is not allowed.
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0: The master operates in 7-bit addressing mode,
1: The master operates in 10-bit addressing mode
0: Master requests a write transfer.
1: Master requests a read transfer.
In 7-bit addressing mode (ADD10 = 0):
SADD[7:1] should be written with the 7-bit slave address to be sent. The bits SADD[9],
SADD[8] and SADD[0] are don't care.
In 10-bit addressing mode (ADD10 = 1):
SADD[9:0] should be written with the 10-bit slave address to be sent.
RM0432 Rev 6
RM0432