Ok, if use 1k5 we get 3.1V reference. Looking at the SAM3X8E datasheet it says the max voltage is 4.0V on any input pin.
Now you remind me I have got a set of stackable headers somewhere, I will try those. ISTR there are plenty of these on ebay quite cheap.
Now you remind me I have got a set of stackable headers somewhere, I will try those. ISTR there are plenty of these on ebay quite cheap.