# STM32 I2C - do I need external pullups?

Do I need external pullups to operate the I2C peripheral on an STM32L412? I was assuming the internal pullups for the GPIO would be sufficient but the communications look very slow (pictured). It seems like the ACK bit is there (9th clock pulse low), but nothing happens afterward.

• You can also use a chip like LTC1694 Oct 5 '20 at 13:44

Yes, you need external pull up resistors. The typical pull up value for the STM32 microcontrollers is 40 k$$\\Omega\$$ (assuming they're even active). You can calculate a correct value, but if you just want to get working, a 4.7 k$$\\Omega\$$ resistor will get you started.