I2C on STM32L151CC dies intermittently

Probably going to want to get it on a logic-analyzer and figure out what's going awry. Have the STM32 trigger when it senses and issue, or misses a NACK.

Make sure you have suitable pull-up resistors.

If the slave device is getting confused or into an odd state it might lock up the bus. ie clamping SDA low.

You might want to add code to clock the slave device out of such a condition, and then try to re-establish communications.

Could relate to the speed at which you're communicating, or the quality of the signal edges, etc. Or potential conflicting with other slaves on the bus, or attempts by multiple threads to use the I2C bus concurrently rather than an orderly fashion.

verify pins aren’t in use by other peripherals on the board. Disconnect the slave to see if it could be misbehaving. If so, cycling SCL 9x on startup will clear it.

By inserting a series resistor one order of magnitude smaller than the pullup, you can find out which party pulls down SDA.

As Clive said, the older I2C module has more quirks. ST does not always propagate a known erratum to errata sheets of all models where given module is used, so you may want to read errata of other STM32 too.

I grew tired of trying to rely on the documentation and work around all individual issues, published or not. I simply use timeout and restart I2C module and the bus when it happens. I've already used bitbanged master, too.

JW