28.6.3.1.3 Adding a Descriptor Between Existing Descriptors

To insert a new descriptor 'C' between two existing descriptors ('A' and 'B'), the descriptor currently executed by the DMA must be identified.

  1. If DMA is executing descriptor B, descriptor C cannot be inserted.
  2. If DMA has not started to execute descriptor A, follow the steps:
    1. Set the descriptor A VALID bit to '0'.
    2. Set the DESCADDR value of descriptor A to point to descriptor C instead of descriptor B.
    3. Set the DESCADDR value of descriptor C to point to descriptor B.
    4. Set the descriptor A VALID bit to '1'.
  3. If DMA is executing descriptor A:
    1. Apply the software suspend command to the channel and
    2. Perform steps 2.1 through 2.4.
    3. Apply the software resume command to the channel.