22.6.2.3 Transfer Descriptors

The transfer descriptors, together with the channel configurations, decide how a block transfer should be executed. Before a DMA channel is enabled (CHCTRLA.ENABLE is written to one) and receives a transfer trigger, its first transfer descriptor must be initialized and valid (BTCTRL.VALID). The first transfer descriptor describes the first block transfer of a transaction.

All transfer descriptors must reside in SRAM. The addresses stored in the Descriptor Memory Section Base Address (BASEADDR) and Write-Back Memory Section Base Address (WRBADDR) registers tell the DMAC where to find the descriptor memory section and the write-back memory section.

The descriptor memory section is where the DMAC expects to find the first transfer descriptors for all DMA channels. As BASEADDR points only to the first transfer descriptor of channel ‘0’ (see figure below). All first transfer descriptors must be stored in a contiguous memory section, where the transfer descriptors must be ordered according to their channel number. For further details on linked descriptors, refer to Linked Descriptors.

The write-back memory section is where the DMAC stores the transfer descriptors for the ongoing block transfers. WRBADDR points to the ongoing transfer descriptor of channel ‘0’. All ongoing transfer descriptors are stored in a contiguous memory section where the transfer descriptors are ordered according to their channel number. The figure below shows an example of linked descriptors on DMA channel ‘0’. For additional information on linked descriptors, refer to the Linked Descriptors.

Figure 22-3. Memory Sections

The size of the descriptor and write-back memory sections are dependent on the number of the most significant enabled DMA channel m, as shown below:

S i z e = 128 bits ⋅ ( m + 1 )

For memory optimization, it is recommended to use the less significant DMA channels, if not all channels are required.

The descriptor and write-back memory sections can either be two separate memory sections, or they can share a memory section (BASEADDR=WRBADDR). The benefit of having them in two separate sections, is that the same transaction for a channel can be repeated without having to modify the first transfer descriptor. In addition, the latency from fetching the first descriptor of a transaction to the first burst transfer is executed, is reduced.