ATmega16U4 fuses & lock bits
The fuse bytes set the clock, start-up, brown-out and boot options; the lock bits protect flash and EEPROM. A fuse bit is programmed at 0 . Change a setting and the byte below updates; the starting values are the factory defaults.
CKDIV8 Divide clock by 8 internally 1 · unprogrammed 0 · programmed CKOUT Clock output on PORTC7 1 · unprogrammed 0 · programmed SUT_CKSEL Select Clock Source Ext. Clock; Start-up time: 6 CK + 0 ms Int. RC Osc.; Start-up time: 6 CK + 0 ms Ext. Low-Freq. Crystal; Start-up time: 1K CK + 0 ms Ext. Low-Freq. Crystal; Start-up time: 32K CK + 0 ms Ext. Low-Freq. Crystal; Start-up time: 1K CK + 0 ms; Int. Cap. Ext. Low-Freq. Crystal; Start-up time: 32K CK + 0 ms; Int. Cap. Ext. Crystal Osc. 0.4-0.9 MHz; Start-up time: 258 CK + 4.1 ms Ext. Crystal Osc. 0.4-0.9 MHz; Start-up time: 1K CK + 65 ms Ext. Crystal Osc. 0.9-3.0 MHz; Start-up time: 258 CK + 4.1 ms Ext. Crystal Osc. 0.9-3.0 MHz; Start-up time: 1K CK + 65 ms Ext. Crystal Osc. 3.0-8.0 MHz; Start-up time: 258 CK + 4.1 ms Ext. Crystal Osc. 3.0-8.0 MHz; Start-up time: 1K CK + 65 ms Ext. Crystal Osc. 8.0- MHz; Start-up time: 258 CK + 4.1 ms Ext. Crystal Osc. 8.0- MHz; Start-up time: 1K CK + 65 ms Ext. Clock; Start-up time: 6 CK + 4.1 ms Int. RC Osc.; Start-up time: 6 CK + 4.1 ms Ext. Low-Freq. Crystal; Start-up time: 1K CK + 4.1 ms Ext. Low-Freq. Crystal; Start-up time: 32K CK + 4.1 ms Ext. Low-Freq. Crystal; Start-up time: 1K CK + 4.1 ms; Int. Cap. Ext. Low-Freq. Crystal; Start-up time: 32K CK + 4.1 ms; Int. Cap. Ext. Crystal Osc. 0.4-0.9 MHz; Start-up time: 258 CK + 65 ms Ext. Crystal Osc. 0.4-0.9 MHz; Start-up time: 16K CK + 0 ms Ext. Crystal Osc. 0.9-3.0 MHz; Start-up time: 258 CK + 65 ms Ext. Crystal Osc. 0.9-3.0 MHz; Start-up time: 16K CK + 0 ms Ext. Crystal Osc. 3.0-8.0 MHz; Start-up time: 258 CK + 65 ms Ext. Crystal Osc. 3.0-8.0 MHz; Start-up time: 16K CK + 0 ms Ext. Crystal Osc. 8.0- MHz; Start-up time: 258 CK + 65 ms Ext. Crystal Osc. 8.0- MHz; Start-up time: 16K CK + 0 ms Ext. Clock; Start-up time: 6 CK + 65 ms Int. RC Osc.; Start-up time: 6 CK + 65 ms Ext. Low-Freq. Crystal; Start-up time: 1K CK + 65 ms Ext. Low-Freq. Crystal; Start-up time: 32K CK + 65 ms Ext. Low-Freq. Crystal; Start-up time: 1K CK + 65 ms; Int. Cap. Ext. Low-Freq. Crystal; Start-up time: 32K CK + 65 ms; Int. Cap. Ext. Crystal Osc. 0.4-0.9 MHz; Start-up time: 1K CK + 0 ms Ext. Crystal Osc. 0.4-0.9 MHz; Start-up time: 16K CK + 4.1 ms Ext. Crystal Osc. 0.9-3.0 MHz; Start-up time: 1K CK + 0 ms Ext. Crystal Osc. 0.9-3.0 MHz; Start-up time: 16K CK + 4.1 ms Ext. Crystal Osc. 3.0-8.0 MHz; Start-up time: 1K CK + 0 ms Ext. Crystal Osc. 3.0-8.0 MHz; Start-up time: 16K CK + 4.1 ms Ext. Crystal Osc. 8.0- MHz; Start-up time: 1K CK + 0 ms Ext. Crystal Osc. 8.0- MHz; Start-up time: 16K CK + 4.1 ms Ext. Crystal Osc. 0.4-0.9 MHz; Start-up time: 1K CK + 4.1 ms Ext. Crystal Osc. 0.4-0.9 MHz; Start-up time: 16K CK + 65 ms Ext. Crystal Osc. 0.9-3.0 MHz; Start-up time: 1K CK + 4.1 ms Ext. Crystal Osc. 0.9-3.0 MHz; Start-up time: 16K CK + 65 ms Ext. Crystal Osc. 3.0-8.0 MHz; Start-up time: 1K CK + 4.1 ms Ext. Crystal Osc. 3.0-8.0 MHz; Start-up time: 16K CK + 65 ms Ext. Crystal Osc. 8.0- MHz; Start-up time: 1K CK + 4.1 ms Ext. Crystal Osc. 8.0- MHz; Start-up time: 16K CK + 65 ms HIGH fuse 0x99 default 0x99 OCDEN On-Chip Debug Enabled 1 · unprogrammed 0 · programmed JTAGEN JTAG Interface Enabled 1 · unprogrammed 0 · programmed SPIEN Serial program downloading (SPI) enabled 1 · unprogrammed 0 · programmed WDTON Watchdog timer always on 1 · unprogrammed 0 · programmed EESAVE Preserve EEPROM through the Chip Erase cycle 1 · unprogrammed 0 · programmed BOOTSZ Select Boot Size Boot Flash size=2048 words start address=$1800 Boot Flash size=1024 words start address=$1C00 Boot Flash size=512 words start address=$1E00 Boot Flash size=256 words start address=$1F00 BOOTRST Boot Reset vector Enabled 1 · unprogrammed 0 · programmed EXTENDED fuse 0xFB default 0xFB HWBE Hardware Boot Enable 1 · unprogrammed 0 · programmed BODLEVEL Brown-out Detector trigger level Brown-out detection at VCC=4.3 V Brown-out detection at VCC=3.5 V Brown-out detection at VCC=3.4 V Brown-out detection at VCC=2.6 V Brown-out detection at VCC=2.4 V Brown-out detection at VCC=2.2 V Brown-out detection at VCC=2.0 V Brown-out detection disabled; [BODLEVEL=111] Lock bits 0xFF default 0xFF BLB1 Boot Loader Protection Mode LPM and SPM prohibited in Boot Section LPM prohibited in Boot Section SPM prohibited in Boot Section No lock on SPM and LPM in Boot Section BLB0 Boot Loader Protection Mode LPM and SPM prohibited in Application Section LPM prohibited in Application Section SPM prohibited in Application Section No lock on SPM and LPM in Application Section LB Memory Lock Further programming and verification disabled Further programming disabled No memory lock features enabled