Synopsys USB OTG Controller (2.65a) occasionally truncates isochronous IN in USB device mode

Question

I'm using a Synopsys OTG core in device mode. Programming an isochronous IN high speed endpoint (USB 2.0) for the maximum transfer per microframe (3 packets of 1024 bytes) using a periodic FIFO dedicated to this endpoint. It works 99+% of the time. But occasionally the transfer is truncated. For example, the first 1024 bytes will go onto the bus with the DATA0 PID (instead of the correct DATA2 PID) and the remaining 2048 bytes will not be sent. Since I've programmed the packet count, multicount, max packet size and transfer size correctly I'm not sure what is causing this.

Obviously this is a very specific question and I don't have much hope of getting an answer, but I figured a shot in the dark was worth a try. Thanks in advance.

Answer 1

Answering my own question in case it may help someone else. It appears this OTG controller has a bug where the TX FIFO does not always empty properly. I found a successful workaround is to flush the FIFO after each TX. It's quick and the truncation symptom goes away.

Answer 2

Isochronous transfers does not guarantee packet delivery. So if host controller has other active transfers, it will silently drop isochronous packets. If you need guaranteed packed delivery, you should use bulk transfers (but then it will not guarantee delivery time).

Isochronous is ideal for applications, like sound or video streaming, where you need constant delivery time, but loss of some frames is ok.

The specification places limits on the bus, allowing no more than 90% of any frame to be allocated for periodic transfers (Interrupt and Isochronous) on a full speed bus. On high speed buses this limitation gets reduced to no more than 80% of a microframe can be allocated for periodic transfers. (c) http://www.beyondlogic.org/usbnutshell/usb4.shtml