I have a FRDM-K22F wired to a Teensy Audio Shield which is essentially an SGTL5000.
Whenever I attempt an I2C write I get a strange result - instead of getting an ACK I get a NACK. Checking with a logic analyser I can see that the I2C SDA line is pulled down only halfway. See the attached screenshot showing the analog output of Channel 2 which is I2C SDA - the problem point is marked at 1.902 V. What could be causing this strange behaviour?
I'm pretty sure it has to do with my K22F board configuration - perhaps I need to disable a pull up resistor somewhere?