40.6.8 Transfer With DMA

USB packets of any length may be transferred when required by the UDPHS device. These transfers always feature sequential addressing.

Packet data system bus bursts may be locked on a DMA buffer basis for drastic overall system bus bandwidth performance boost with paged memories. These clock-cycle consuming memory row (or bank) changes will then likely not occur, or occur only once instead of several times, during a single big USB packet DMA transfer in case another system bus host addresses the memory. The locked bursts result in up to 128-word single-cycle unbroken system bus bursts for bulk endpoints and 256-word single-cycle unbroken bursts for isochronous endpoints.

This maximum burst length is then controlled by the lowest programmed USB endpoint size (EPT_SIZE field in the UDPHS_EPTCFGx register) and DMA Size (BUFF_LENGTH field in the UDPHS_DMACONTROLx register).

The USB 2.0 device average throughput may be up to nearly 60 Mbyte/s. Its internal client average access latency decreases as burst length increases due to the 0 wait-state side effect of unchanged endpoints. If at least 0 wait-state word burst capability is also provided by the external DMA system bus clients, each of both DMA system busses need less than 50% bandwidth allocation for full USB 2.0 bandwidth usage at 30 MHz, and less than 25% at 60 MHz.

The UDPHS DMA Channel Transfer Descriptor is described in the section UDPHS DMA Channel Transfer Descriptor.