The maximum clock frequency that can be used with the internal oscillator is 16 MHz, but it's recommended to use 12 MHz or lower for reliable operation.
To use an external crystal oscillator, connect the crystal to the XIN and XOUT pins, and configure the UCSCTL6 register to select the external crystal oscillator as the clock source.
The minimum voltage required for the MSP430G2131IN14 to operate is 1.8V, but it's recommended to use a voltage of 2.2V or higher for reliable operation.
To use the built-in UART module, configure the UCAxCTL0 register to select the UART mode, set the baud rate using the UCAxBR0 and UCAxBR1 registers, and use the UCAxTXBUF and UCAxRXBUF registers to transmit and receive data.
The maximum current that can be sourced or sunk by the GPIO pins is 4mA, but it's recommended to limit the current to 2mA or less to ensure reliable operation.