user47634: I have a project where I need to measure 0 - 5 V. I’m using a ADS1115 to have a better precision, connected to a ESP12. Since the ESP is only 3.3 V tolerant and the ADS powered by 5 V (because it needs to be powered a the same voltage as the measured voltage), I put the two pull-up resistor of the I2C bus on 3.3 V. 
Is it safe to operate like this?

user4143: Is your arrangement safe? Yes, perfectly.
Is your arrangement a good idea? No, it is not.
When the ADS1115 is run at 5V it needs at least 3.5V on the I2C bus to properly register a HIGH.
However you can pull up to 5v instead and you won't have a problem.  Not only is the ESP8266 tolerant of 5v, but that 5v is severely limited in current by the pull-up resistors, so even non tolerant inputs would have a hard time complaining. Worst case scenario, the ESP8266 pulls the bus down to maybe 3.8v (3.3v plus an ESD diode drop) which is fine for a logic high on the ADS1115.
However, if you want to do things properly, you really should use a proper I2C level translator.

47634: <p>I have a project where I need to measure 0 - 5&nbsp;V. I’m using a ADS1115 to have a better precision, connected to a ESP12. Since the ESP is only 3.3&nbsp;V tolerant and the ADS powered by 5&nbsp;V (because it needs to be powered a the same voltage as the measured voltage), I put the two pull-up resistor of the I2C bus on 3.3&nbsp;V. </p>

<p>Is it safe to operate like this?</p>
 4143: <p>Is your arrangement safe? Yes, perfectly.</p>
<p>Is your arrangement a good idea? No, it is not.</p>
<p>When the ADS1115 is run at 5V it needs at least 3.5V on the I2C bus to properly register a HIGH.</p>
<p>However you can pull up to 5v instead and you won't have a problem.  Not only is the ESP8266 tolerant of 5v, but that 5v is severely limited in current by the pull-up resistors, so even non tolerant inputs would have a hard time complaining. Worst case scenario, the ESP8266 pulls the bus down to maybe 3.8v (3.3v plus an ESD diode drop) which is fine for a logic high on the ADS1115.</p>
<p>However, if you want to do things properly, you really should use a proper I2C level translator.</p>
