It can't have been "right out" of the bag as you must have programmed it first?YES, you are correct.It is unusual that high voltage appears to stop it from working. Did you notice any difference in current draw? As if there was one component drawing 'too much' current at high voltage or some subsystem that was shut down at high voltage then became active as the voltage then came back into the acceptable range? Did you notice any variation on the 3.3V rail in these two conditions?No I did not see a difference in current but I did notice that it was not dead steady. The current varied around 560 to 580 mA.Which I2C lines were you using? Remember that only one of the two pairs has pullup resistors on the Due board.In this case I was using 20 (SDA) and 21 (SCL). Were you able to see I2C requests going out in the non-working condition (and the slave not hearing or not replying) or was the master I2C silent in that condition? This is best observed with an oscilloscope but a logic analyzer is also good.Well, after a fashion. The slave (uno) is programed to output its data both I2C and Serial. Even when the slave is powered via the 12 jack with no connection to laptop the TX led flashes in sync with the I2C signal.I can borrow a good Fluke oscilliscope but i have never used one before.
how are you powering the slave. Either a Laptop USB or 12V Jack. I am now using DC-DC down converters from CUI On the Due, the 12V power input goes through a 5V regulator before it does anything else. When you power up to do, you should be getting 5V at the 5V pin, And 3.3V at the 3.3V pin. you should also get 12V, at the VIN pin.5V pin = 4.993.3 = 3.25Vin =11.96 And other than the VIN in, There should be no difference between using the USB port or the power port when powering the Due. And unless you are powering the I2c slave unit with the VIN pen, it should work the same. check your voltages on your Due board.
Well, after a fashion. The slave (uno) is programed to output its data both I2C and Serial. Even when the slave is powered via the 12 jack with no connection to laptop the TX led flashes in sync with the I2C signal.