The transmit FIFO (TxFIFO) stores data written from the APB interface until it is removed by the transmit module and loaded into its shift register. The TxFIFO’s maximum data width is eight bits. Data is loaded into the TxFIFO by writing to the TxFIFO register.
When data is loaded into the TxFIFO, the TxFIFO empty flag is cleared and remains in this Low state until the last word in the TxFIFO has been removed and loaded into the transmitter shift register. This means that host software has another full serial word time until the next data is needed, allowing it to react to the empty flag being set and write another word in the TxFIFO without loss in transmission time.
The TxFIFO full interrupt status (TXFULL) indicates that the TxFIFO is completely full and prevents any further data from being loaded into the TxFIFO. If another APB write to the TxFIFO is performed, an overflow is triggered and the write data is not loaded into the TxFIFO. The transmit FIFO nearly full flag (TNFUL) indicates that there is not enough free space in the FIFO for one more write of the programmed size, as controlled by the WSIZE bits of the Mode register.
The TxFIFO nearly-full flag (TNFUL) indicates that there is only byte free in the TxFIFO.
A threshold trigger (TTRIG) can be setup on the TxFIFO fill level. The Transmitter Trigger register can be used to setup this value, such that the trigger is set when the TxFIFO fill level reaches this programmed value.