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HID Report Descriptor Contains Wrong Byte

Question asked by Joshua on Apr 9, 2014
Latest reply on Apr 9, 2014 by Joshua
I've got a USB HID device based on a STM32F427 that can be set up to run as either FS or HS USB. When I use FS, all is well. However, when I use HS (which uses the same descriptor), the report descriptor contains one extra byte when I view the USB traffic using my analyzer: a 0x00 as the first byte of the descriptor. The rest of the descriptor is 100% correct, except shifted right by that one byte. Since the report size is fixed, this causes me to lose a byte off the back end of the descriptor report, which then results in the host setting the device to unconfigured.

I have stepped through where the descriptor is sent, and the pointer to the report buffer is correct all of the way through transmitting the data over USB via the ST library function DCD_EP_Tx. I have verified that my report is correct, and located at that address. My only clue is that when I view the Memory contents in my IDE at the starting address of the descriptor, I do have a 0x00 located just before the start of my descriptor. However, as the pointer is always correct, it's not clear to me how this "extra" byte is getting transmitted, or if the previous memory location is indeed the source of this extra byte.

Anyone have any thoughts?