Forum Post: RE: JTAG security fuse blown and cannot get into BSL using USB port by pressing switch

Section 7.3.1 of the User's Guide says: The logical value of an erased flash memory bit is 1. Each bit can be programmed from 1 to 0 individually, but to reprogram from 0 to 1 requires an erase cycle. The smallest amount of flash that can be erased is one segment. I'd guess you tried to program a 1 bit without erasing, or you erased a BSL segment without re-programming the BSL code in it.