SWCU191 February 2022 CC1311P3 , CC1311R3 , CC2651P3 , CC2651R3 , CC2651R3SIPA
The common TX FIFO is a 16 bit wide, 8 location deep, first-in first-out memory buffer. The CPU writes data to the FIFO by writing the SSI Data register, SSI:DR (see Section 23.4.1), and data is stored in the FIFO until it is read out by the transmission logic.
When configured as a master or a slave, parallel data is written into the TX FIFO before serial conversion and transmission to the attached slave or master, respectively, through the SSIn_TX pin.
In slave mode, the SSI transmits data each time the master initiates a transaction. If the TX FIFO is empty and the master initiates transaction, the slave transmits zeroes. User or software is responsible to make valid data available in the FIFO as needed. The SSI can be configured to generate an interrupt when the FIFO is half full (< 4 words), or a µDMA request when the FIFO is not FULL.