When the USB host wants to receive isochronous (ISO) data, it issues an IN token on the isochronous endpoint.
After the data has been transferred using the EasyDMA, the USB controller on the isochronous IN endpoint responds to the IN token with the transferred data using the ISOIN.MAXCNT for the size of the packet.
The ISO IN data endpoint has to be explicitly enabled by software through the ISOIN0 bit in register EPINEN.
When an ISO IN endpoint is enabled and no data transferred with EasyDMA, the response of the USBD depends on the setting of the RESPONSE field in register ISOINCONFIG. It can either provide no response to an IN token or respond with a zero-length data.
If the EasyDMA transfer on the isochronous endpoint is not completed before the next SOF event, the result of the transfer is undefined.
The maximum size of an ISO IN transfer in USB full-speed is 1023 bytes. The data buffer has to be a multiple of 4 bytes 32-bit aligned in memory. However, the amount of bytes transferred on the USB data endpoint can be of any size (up to 1023 bytes, if not shared with an OUT ISO endpoint).