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By default the `i2c_master` driver for ChibiOS uses the B6 pin for
`I2C1_SCL` and the B7 pin for `I2C1_SDA`. However, the ChibiOS board
file used for the F401 Blackpill board (`ST_STM32F401C_DISCOVERY`)
configures B6 as `I2C1_SCL` and B9 as `I2C1_SDA`, and if that
configuration is left unchanged, enabling the `i2c_master` driver
results in having two pins (B7 and B9) configured as `I2C1_SDA` at the
same time, which does not work properly (experimental results show that
the B9 pin still works as `I2C1_SDA` in that case, and the B7 pin does
not work).
Configure the B9 pin as an input with pull-up in `board_init()`, so that
the B7 pin can be configured as `I2C1_SDA` by the I2C driver.
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