24.7.3 Transmitter Flags and Interrupts

The USART transmitter has two flags that indicate its state: USART Data Register Empty (UDRE) and Transmit Complete (TXC). Both flags can be used for generating interrupts.

The Data Register Empty (UDRE) flag indicates whether the transmit buffer is ready to receive new data. This bit is set when the transmit buffer is empty and cleared when the transmit buffer contains data to be transmitted that has not yet been moved into the Shift Register. For compatibility with future devices, always write this bit to zero when writing the UCSRnA register.

When the Data Register Empty Interrupt Enable (UDRIE) bit in UCSRnB is written to '1', the USART Data Register Empty Interrupt will be executed as long as UDRE is set (provided that global interrupts are enabled). UDRE is cleared by writing UDRn. When interrupt-driven data transmission is used, the Data Register Empty interrupt routine must either write new data to UDRn in order to clear UDRE or disable the data register empty interrupt - otherwise, a new interrupt will occur once the interrupt routine terminates.

The Transmit Complete (TXC) flag bit is set when the entire frame in the Transmit Shift register has been shifted out and there are no new data currently present in the transmit buffer. The TXC flag bit is either automatically cleared when a transmit complete interrupt is executed, or it can be cleared by writing a '1' to its bit location. The TXC flag is useful in half-duplex communication interfaces (like the RS-485 standard), where a transmitting application must enter receive mode and free the communication bus immediately after completing the transmission.

When the Transmit Complete Interrupt Enable (TXCIE) bit in UCSRnB is written to '1', the USART transmit complete interrupt will be executed when the TXC flag becomes set (provided that global interrupts are enabled). When the transmit complete interrupt is used, the interrupt handling routine does not have to clear the TXC flag, this is done automatically when the interrupt is executed.