I use the following fuse settings. L=0XFF H=0XDE Ex=0X05 Lock=0X0F
However, after setting the fuses, if I exit the fuse settings page an return again, the Low and High are correct but the Extended has changed to 0XFD? Also the lockbit has changed to 0XFF?
The Extended fuse setting 0xFD and 0x05 are the same thing. Only the lower 3 bits are significant. The upper 5 bits are undefined, so they will read back as unprogrammed.
If I burn the bootloader first and then set the fuses and lockbits, after exiting and reading fuses again fuses are as above but the lock bit reads 0XCF. Why would these change?
The Lock bits 0xCF and 0x0F are the same also. Again only the lower 6-bits are significant for the lock bits. The upper 2 bits are undefined so they will also read back as unprogrammed.