38.6.4 USB DMA Operation

USB packets of any length may be transferred when required by the USBHS. These transfers always feature sequential addressing. Such characteristics mean that in case of high USBHS throughput, both AHB ports benefit from “incrementing burst of unspecified length” since the average access latency of AHB Clients can then be reduced.

The DMA uses word “incrementing burst of unspecified length” of up to 256 beats for both data transfers and channel descriptor loading. A burst may last on the AHB busses for the duration of a whole USB packet transfer, unless otherwise broken by the AHB arbitration or the AHB 1-Kbyte boundary crossing.

Packet data AHB bursts may be locked on a DMA buffer basis for drastic overall AHB bus bandwidth performance boost with paged memories. This prevents large AHB bursts from being broken in case of conflict with other AHB bus Hosts, thus avoiding access latencies due to memory row changes. This means up to 128 words single cycle unbroken AHB bursts for bulk pipes/endpoints and 256 words single cycle unbroken bursts for isochronous pipes/endpoints. This maximal burst length is then controlled by the lowest programmed USB Pipe/Endpoint Size (USBHS_HSTPIPCFGx.PSIZE / USBHS_DEVEPTCFGx.EPSIZE) and the Buffer Byte Length (USBHS_HSTDMACONTROLx.BUFF_LENGTH / USBHS_DEVDMACONTROLx.BUFF_LENGTH) fields.

The USBHS average throughput can reach nearly 480 Mbps. Its average access latency decreases as burst length increases due to the zero wait-state side effect of unchanged pipe/endpoint. Word access allows reducing the AHB bandwidth required for the USB by four, as compared to native byte access. If at least 0 wait-state word burst capability is also provided by the other DMA AHB bus Clients, each DMA AHB bus needs less than 60% bandwidth allocation for full USB bandwidth usage at 33  MHz, and less than 30% at 66  MHz.