USB 2.0 allows a Device to NAK a USB In request if it has no data to send. This is normal. We were using this on code that was originally developed on a TI Stellaris processor, then ported to the K51 when TI suddenly discontinued the Stellaris line.
Reviewing the K51 Sub-Family Reference Manual, I find no reference whatsoever to forcing a NAK condition. Reviewing the USB code, I find no reference whatsoever to NAK in Device mode.
How do I force a NAK if I have nothing to send?