Reference manual RM0383, "STM32F411xC/E advanced ARM®-based 32-bit MCUs", says:18.6.7 I2C Status register 2 (I2C_SR2) Note: Reading I2C_SR2 after reading I2C_SR1 clears the ADDR flag, even if the ADDR flag wasset after reading I2C_SR1. Consequently, ...