AN75779 with ISOC endpoint does not work | Cypress Semiconductor
AN75779 with ISOC endpoint does not work
It is possible to modify the AN75779 example to use ISOC IN endpoint to transfer image data to the USB host. It should be noted that the maximum bandwidth that can be reached is 24MBps in USB2.0 and 8MBps in USB3.0 with standard UVC driver. Please refer to http://www.cypress.com/?app=forum&id=167&rID=69881 for explanation. So Ideally, this should be used only for USB2.0. The following changes assume USB2.0 implementation.
First modification needed is in the USB descriptors to report ISOC endpoints as compared to bulk endpoints.
Second modification needed is in the alternate settings. Currenlty in AN75779, the video streaming interface has only one alternate setting. For ISOC implementation, the alternate setting 0 needs to report zero bandwidth and alternate setting 1 should report a streaming ISOC endpoint.
Third modification required is in the endpoint configuration. The streaming endpoint is set as bulk by setting endPointConfig.isoPkts = 0 in the application init function. It sould be set to 1 and the mult setting should be set to the desired value depending on bandwidth required.
Fourth modification is to change the buffer size. Isoc implementation of UVC requires UVC header every micro frame. so the buffer size needs to be set to 1K * iso mult with header offset as in AN75779. I would recommend that the mult should be set to 3 even if full 24MBps is not required which will imply a buffer size of 3072 bytes.
Fifth modification: Different image resoutions and bit depths cause different size of data payloads in the end of frame buffer. The post http://www.cypress.com/?app=forum&id=167&rID=69873 talks about USB2.0 isoc endpoint issue in FX3. If this issue is encountered with a particular image size/bit depth setting, the size of the buffer can be reduced by a certain calculated amount such that the end of frame buffer would always have more than 2048 bytes to transfer.
Sixth modification is to set the streaming event when alternate setting is not equal to 0 in the set interface command. Unlike the bulk implementation in AN75779 where the UVC host starts streaming after commit control request, the isoc implementation of UVC starts streaming video after set interface with alternate setting equals non zero value based on USB descriptor.