I am trying to read a pH level from an Atlas Scientific pH circuit using I2C. I am able to read values with an Arduino Uno with no problems, but I get garbage data when reading with a Spark Core. Some googling suggested that I need to add pullup resistors to the SDA and SCL lines. Looking through the documentation I noticed that I can activate internal pullup resistors. I tried various API calls to activate the internal pullup resistor, but never got it working. Finally I added external 10k ohm resistors to the SDA and SCL lines and my program stated working as expected (see hardware setup below). While I am happy that my circuit works as expected, I am still perplexed as to why the internal pullup resistors did not work. Has anyone got the internal pullup resistors working with I2C communication?