35.2.1.4 Transmit Interrupt Flag

The UxTXIF Interrupt Flag bit in the PIR register is set whenever the UART transmitter is enabled and no character is being held for transmission in the UxTXB register. In other words, the UxTXIF bit is clear only when the TSR is busy with a character and a new character has been queued for transmission in the UxTXB register.

The UxTXIF interrupt is enabled by setting the UxTXIE Interrupt Enable bit in the PIE register. However, the UxTXIF Flag bit will be set whenever the UxTXB register is empty, regardless of the state of the UxTXIE Enable bit. The UxTXIF bit is read-only and cannot be set or cleared by software.

To use interrupts when transmitting data, set the UxTXIE bit only when there is more data to send. Clear the UxTXIE Interrupt Enable bit upon writing the UxTXB register with the last character of the transmission.