The FSI transmitter has a 16-word b

The FSI transmitter has a 16-word buffer from which it pulls data to transmit. This buffer is implemented as a circular buffer, not a FIFO, so some care must be taken to properly interpret buffer overrun and underrun, as well as the TX_BUF_PTR_STS register. These flags and pointers work under the assumption that the software or DMA is using the buffer as a circular buffer. This mode of operation is the only way in which the overrun, underrun, and pointer status are meaningful. If data is being sourced by the DMA and there is some other periodic trigger mechanism trying to initiate transfers, underrun becomes a critical error. If an underrun happens, it means that the buffer went out of sync. This would not only affect the current transfer, but all future transfers also could not be guaranteed due to the ring buffer. Under such conditions, the underrun would need a soft reset to cleanly recover. Alternately, the software could manually stop the transmitting, reset the buffer pointers, clear the remaining error conditions, and then restart transmission. The software method involves a few steps, while the soft reset is a single action and will guarantee a full reset of the control registers.