SNIU028D February 2016 – September 2020 UCD3138 , UCD3138064 , UCD3138064A , UCD3138128 , UCD3138A , UCD3138A64
This section describes how a CPU interrupt can be initiated by a transmit ready condition.
The transmit ready (TXRDY) flag is set when the UART transfers the contents of UARTTXBUF to the shift register, UARTTXSHF. The TXRDY flag indicates that UARTTXBUF is ready to be loaded with more data.
In addition, the UART sets the TX EMPTY bit if both the UARTTXBUF and UARTTXSHF registers are empty.
Transmit interrupts are enabled by the TX_INT_ENA (UARTCTRL3.3) bit. If the TX_INT_ENA bit (UARTCTRL3.3) is set, then a transmit interrupt is generated when the TXRDY flag goes high.
Writing data to the UARTTXBUF register clears the TXRDY bit. When this data has been moved to the UARTTXSHF register, the TXRDY bit is set again.
The interrupt request can be suspended by clearing the TX_INT_ENA bit; however, when the TX_INT_ENA bit is again set to 1, the TXRDY interrupt is asserted again.
The transmit interrupt request can be eliminated until the next series of values is written to UARTTXBUF by disabling the transmitter via the TXENA bit (UARTTXST.0 = 0), an UART software reset, or by a device hardware reset.