22.6.3.1.2 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 (DESCADDR)
   • Set the destination address (DESCADDR)
   • Set the source address (SRCADDR)
   • Configure the block transfer control (BTCTRL) 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 DESCADDR from the write-back memory.
   • If the DMA has not already fetched the descriptor that requires changes (in other words, DESCADDR is wrong):
     • Update the DESCADDR 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 that requires changes:
     • Set the Channel Suspend Software command and wait for the suspend interrupt
     • Update the next descriptor address (DESCADDR) in the write-back memory
     • Clear the interrupt sources and set the Resume Software command
     • Update the DESCADDR 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.