26.4.3.1.2 Modifying a Descriptor in a List

In order to add descriptors to a linked list, the following actions must be performed:

1. Enable the Suspend interrupt for the DMA channel.
2. Enable the DMA channel.
3. Reserve memory space in SRAM to configure a new descriptor.
4. Configure the new descriptor:
   • Set the next descriptor address in the Next Descriptor Address (DESCADDR) register
   • Set the destination address in the Block Transfer Destination Address (DSTADDR) register
   • Set the source address in the Block Transfer Source Address (SRCADDR) register
   • Configure the Block Transfer Control (BTCTRL) register, including
     • Optionally enable the Suspend block action
     • Set the descriptor VALID bit
5. Clear the VALID bit for the existing list and for the descriptor which has to be updated.
6. Read Next Descriptor Address (DESCADDR) register from the Write-Back memory.
   • If the DMA has not already fetched the descriptor which requires changes (i.e., DESCADDR is wrong):
     • Update the Next Descriptor Address (DESCADDR) register location of the descriptor from the List
     • Optionally clear the Suspend block action
     • Set the descriptor VALID bit to '1'
     • Optionally enable the Resume software command
   • If the DMA is executing the same descriptor as the one which requires changes:
     • Set the Channel Suspend software command and wait for the Suspend interrupt
     • Update the Next Descriptor Address (DESCADDR) register in the write-back memory
     • Clear the interrupt sources and set the Resume software command
     • Update the Next Descriptor Address (DESCADDR) register location of the descriptor from the List
     • Optionally clear the Suspend block action
     • Set the descriptor VALID bit to '1'
7. Go to step 4 if needed.