SLAU723A October 2017 – October 2018 MSP432E401Y , MSP432E411Y
The DMA may be used in connection with any type of transfer, but it is particularly useful when large blocks of data are to be transferred through a Bulk endpoint. The USB protocol requires that large data blocks are transferred by sending a series of packets of the maximum packet size for the endpoint (512 bytes for high speed, 64 bytes for full speed). The last packet in the series may be less than the maximum packet size. The receiver may use the reception of this short packet to signal the end of the transfer (a null packet may be sent at the end of the series if the size of the data block is an exact multiple of the maximum packet size). The DMA may be used in either device mode or host mode to avoid the overhead of having to interrupt the processor after each individual packet, and instead, only interrupting the processor after the transfer has completed. The following sections outline the basic actions that are involved in using the DMA alongside some standard types of Bulk Tx and Bulk Rx transfers.