Does MSP430 and AVR have internal pull-up on UART's RXD line?
Find the datasheet of the device
Figure out which pins have the UART functionality
Figure out if the output drivers for these pins support pull up / down resistors
Check the text of the I/O pin drivers in the datasheet if any exceptions are made for UART mode


For AVR all pins support pull up (except Vcc, GND and except RESET to some extent) you can turn on pull up resistors by setting Data Direction Register for a pin as input and enable the pull up by setting the corresponding pin in the PORT register.