I have a 3.3v XMega talking to a 3.3v KL15 over UART1 with a 1k series resistor between the two. On some units, when the XMega drives TX low the KL15 sees about 2v on the RX. If I remove the 1k series resistor the KL15 sees 0V on the RX. If I disconnect the KL15 RX and put a 10k pull-up on the XMega TX it has no problem driving it to 0v.
But this is only on some units. Other units work just fine with the 1k series resistor. The RX on the KL15 goes to 0v.
I didn’t chose the 1k and I don’t think it needs to be there but I would still like to understand what’s going on inside the KL15 that clamps Vin-low to 2v. Why don’t all the KL15’s work this way? 2 identical units, same code, a difference of 2v.