USB_TO_SERIAL.asm in USB to Serial firmware
The CY64x13 hardware automatically generates SE0s. This feature was implemented specifically as a means of preventing babble in hub applications. For the most part, this behavior has been successfully implemented with no ill effects. However, in some host environments and in non-hub applications this might cause some problems. The register located at F2h is an undocumented register. Writing a 10h to register F2h disables the generation of SE0s.