Hi all, I've been checking out the various problems encountered by people using the KL25Z and external I2C devices. I thought I'd share some of what I've learned. My test is a DS1307 realtime clock IC. This requires a 5V PSU and thence generates 5V I2C signals. Using my KL25Z powered from the USB, and driving the DS1307 from its 5V pi I get no I2C connection at all. Sadly this is exactly the configuration that the arduino datalogger shield uses (where my DS1307 is located). If however, I drive te DS1307 from a 3.3V supply (which is not recommended and requires some reworking of the arduino board) it does in fact work ok , even allowing writing of the internal eeprom! Why ? Well, several references across the net suggest that the KL25Z's inputs are not really 5V tolerant - they are pulled down to ground via a diode which can breakdown at this voltage (occasionally). What's a fix ? Probably a bidirectional level shifter like the adafruit one which uses the BSS138 mosfet. {{/media/uploads/cstevens/screenshot_from_2016-12-19_08-39-29.png}} Hope this is useful to people struggling with this IC.