I see the strange behaviour in the emulator: I have installed U0IRQ in my vic and enabled it. Also, i have disabled all types of interrupts in U0IER. But after I send some data to serial with this classic _sendchar():
while(!(U0LSR&0x20)) {} return U0THR = ch;
, the THRE interrupt is generated.
It seems that this behaviour do not appears on hardware.