The transmit interrupt changes state when one of the following events occurs:
- If the FIFOs are enabled and the transmit FIFO is equal to or lower than the programmed trigger level, the transmit interrupt is asserted High. The transmit interrupt is cleared by writing data to the transmit FIFO until it becomes greater than the trigger level, or by clearing the interrupt.
- If the FIFOs are disabled (have a depth of one location) and there is no data present in the transmitters single location, the transmit interrupt is asserted High. It is cleared by performing a single write to the transmit FIFO, or by clearing the interrupt.
- To update the transmit FIFO, write data to the transmit FIFO, either prior to enabling the UART and the interrupts, or after enabling the UART and interrupts.
Note: The transmit interrupt is
based on a transition through a level, rather than on the level itself. When the
interrupt and the UART is enabled before any data is written to the transmit FIFO the
interrupt is not set. The interrupt is only set, after written data leaves the single
location of the transmit FIFO and it becomes empty.