Cyclic CDMA Mode - 4.1 English

AXI Central Direct Memory Access LogiCORE IP Product Guide (PG034)

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4.1 English

AXI CDMA can be run in cyclic mode by making certain changes to the BD chain setup. In cyclic mode, CDMA fetches and processes the same BDs without any interruption. The CDMA continues to fetch and process until it is stopped or reset. To enable cyclic operation, the BD chain should be set up as show in the following figure.

Figure 3-1:      BD Chain

X-Ref Target - Figure 3-1


In this setup the Tail BD points back to the first BD. The Tail Descriptor Register does not serve any purpose and is used only to trigger the CDMA engine. Follow the same programming sequences as mentioned in Scatter Gather Mode. Ensure that the cyclic bit in the control register is set.

Program the Tail Descriptor register with some value which is not a part of the BD chain. Say for example 0x50.

After the Tail Descriptor register is programmed, the CDMA starts fetching and processing the BDs (which are set up in a ring fashion) until the CDMA is stopped or reset.