Intel® Agilex™ Hard Processor System Technical Reference Manual

ID 683567
Date 2/14/2023
Public

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21.5.1. Transmit FIFO Underflow

During UART serial transfers, transmit FIFO requests are made to the DMA controller whenever the number of entries in the transmit FIFO is less than or equal to the decoded level of the Transmit Empty Trigger (TET) field in the FIFO Control Register (FCR), also known as the watermark level. The DMA controller responds by writing a burst of data to the transmit FIFO buffer, of length specified as DMA burst length. †

Data should be fetched from the DMA often enough for the transmit FIFO to perform serial transfers continuously, that is, when the FIFO begins to empty, another DMA request should be triggered. Otherwise, the FIFO runs out of data (underflow) causing a STOP to be inserted on the UART bus. To prevent this condition, you must set the watermark level correctly. †