RM0319 datasheet(126/368 Pages) STMICROELECTRONICS

126 / 368 page

USB 2.0 Device ports (UDC)

RM0319

126/368

Doc ID 022640 Rev 3

Now the DMA transfers the data from the system memory to the relevant endpoint FIFO. As

shown in Figure 36, these endpoint buffers are RAM-based implementations with

programmable sizes. When the USB Host retries with another in token, the UDC-AHB

subsystem provides the data to the UDC reading the endpoint buffers for transmission to

USB Host. When the transmission is complete, the status is written back into the buffer

descriptor’s status quadlet. Then, the subsystem clears the endpoint-specific poll demand

bit once the descriptor chain reaches the last descriptor.

Note:

The application can read the poll demand bit to determine if the descriptor chain is serviced

or not.

In transfers with ISO, endpoints are handled similarly. In this case, the transfer of in data

from the application memory to the endpoint FIFO is not initiated by request tokens from the

USB Host, but the application sets the poll demand bit of CSR as soon as data is available.

At the following bit setting, the UDC-AHB subsystem tags data for isochronous endpoints

with a frame number. The UDC, which maintains the frame counter, sends the isochronous

data in the intended frame, whereas the SOF tracker module (SOF tracker on page 120)

tracks the incoming SOFs and their frame numbers. Three distinct scenarios can be raised

up:

●

if the incoming frame number matches the frame number in the buffer, the UDC is

allowed to transfer the frame from the appropriate data buffer.

●

if the frame number in the SOF is greater than the frame number in the frame counter

of UDC, the DMA module skips the buffers to align to the correct frame number.

●

if the frame number in the SOF is less than the subsystem’s frame number, the DMA

waits for a few frames to align to the correct frame number.

Hooks are provided for the application to flush the subsystem’s FIFOs in case of missing

SOFs. The transaction flow of in data from the USB Host to the application memory is given

in Figure 37.

RM0319 датащи(PDF) 126 Page - STMicroelectronics