1.8V UART with 3.3V UART
Do not use meg ohm resistors for the voltage divider from the 3.3V to 1.8V UART direction flow. Something with resistors more in the 3.3K to 10K range would be much more appropriate.





For the 1.8V to 3.3V operational direction you can easily make a level translator with a couple of transistors and resistors. 2N3906 NPN transistors will work great.