We are having a problem with the MK50DX256CMC10, both 4N22D and 5N22D. The device is completely unresponsive on the JTAG interface, when powered by 1.8V. However when the board voltage is raised to 3.3V, the device operates normally (programs, runs, etc.). We are using the Segger J-link Ultra+ Emulator, which supports 1.8V on the target interface.
Thanks,
Damon