USART without the RX part, now works. Messages are printed to the terminal window.

This is the Arduino code I used for the sketch.

 void setup() {
   Serial.begin(9600); 
 }
 
 void loop() {
   Serial.print("Test UART Message " );
   Serial.println("0123456789");
   
   delay(2);                     
 }

IRQ USART, UDRE "__vector_19" is responsible for writing out the TX buffer to I/O 0xC6

"_ZN5Print5printEPKc" Is the subroutine that gets called to update the TX buffer.