Question: The USB 2.0 specification says that when the USB host sends an IN token to a device, and if the device has no data to return to the host, it should respond with a NAK token. However, the CY7C68001 datasheet says there is no description and no register corresponding to generation of a NAK token. How does one return a NAK token using the SX2? Does the external master have to construct one?
SX2 will respond automatically with a NAK to the host if there is no data in the IN endpoint FIFO. Do not worry about setting a "NAK register" directly. This is the advantage of having a "smart SIE" like SX2.