Intel® Stratix® 10 Hard Processor System Technical Reference Manual

ID 683222
Date 11/28/2022
Public

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19.5.6.2.1. Example 1: Transmit FIFO Watermark Level = 64

Consider the example where the assumption is made: †

DMA burst length = FIFO_DEPTH - DMATDLR

Here the number of data items to be transferred in a DMA burst is equal to the empty space in the transmit FIFO buffer.

Consider the following:

  • Transmit FIFO watermark level = DMATDLR = 64 †
  • DMA burst length = FIFO_DEPTH - DMATDLR = 192 †
  • SPI transmit FIFO_DEPTH = 256 †
  • Block transaction size = 960 †
Figure 107. Transmit FIFO Watermark Level = 64

The number of burst transactions needed equals the block size divided by the number of data items per burst:

Block transaction size/DMA burst length = 960/192 = 5

The number of burst transactions in the DMA block transfer is 5. But the watermark level, DMATDLR, is quite low. Therefore, there is a high probability that the SPI serial transmit line needs to transmit data when there is no data left in the transmit FIFO buffer. This is a transmit underflow condition. This occurs because the DMA has not had time to service the DMA request before the FIFO buffer becomes empty.