32.2 Fuse Bits
The device has three Fuse bytes. The following tables describe briefly the functionality of all the fuses and how they are mapped into the Fuse bytes. Note that the fuses are read as logical zero, “0”, if they are programmed.
| Extended Fuse Byte | Bit No. | Description | Default Value |
|---|---|---|---|
| – | 7 | – | 1 |
| – | 6 | – | 1 |
| – | 5 | – | 1 |
| – | 4 | – | 1 |
| CFD | 3 | Disable Clock Failure Detection | 0 (programmed, CFD disable) |
| BODLEVEL2(1) | 2 | Brown-out Detector trigger level | 1 (unprogrammed) |
| BODLEVEL1(1) | 1 | Brown-out Detector trigger level | 1 (unprogrammed) |
| BODLEVEL0(1) | 0 | Brown-out Detector trigger level | 1 (unprogrammed) |
Note:
- Refer to Table BODLEVEL Fuse Coding in System and Reset Characteristics for BODLEVEL Fuse decoding.
| High Fuse Byte | Bit No. | Description | Default Value |
|---|---|---|---|
| RSTDISBL(1) | 7 | External Reset Disable | 1 (unprogrammed) |
| DWEN | 6 | debugWIRE Enable | 1 (unprogrammed) |
| SPIEN(2) | 5 | Enable Serial Program and Data Downloading | 0 (programmed, SPI programming enabled) |
| WDTON(3) | 4 | Watchdog Timer Always On | 1 (unprogrammed) |
| EESAVE | 3 | EEPROM memory is preserved through the Chip Erase | 1 (unprogrammed), EEPROM not preserved |
| BOOTSZ1 | 2 | Select Boot Size (see Boot Loader Parameters) | 0 (programmed)(4) |
| BOOTSZ0 | 1 | Select Boot Size (see Boot Loader Parameters) | 0 (programmed)(4) |
| BOOTRST | 0 | Select Reset Vector | 1 (unprogrammed) |
Note:
- Refer to Alternate Functions of Port C in I/O-Ports chapter for description of RSTDISBL Fuse.
- The SPIEN Fuse is not accessible in serial programming mode.
- Refer to WDTCSR – Watchdog Timer Control Register for details.
- The default value of BOOTSZ[1:0] results in maximum Boot Size. See table Boot Size Configuration in subsection Boot Loader Parameters in the previous chapter for details.
| Low Fuse Byte | Bit No. | Description | Default Value |
|---|---|---|---|
| CKDIV8(4) | 7 | Divide clock by 8 | 0 (programmed) |
| CKOUT(3) | 6 | Clock output | 1 (unprogrammed) |
| SUT1 | 5 | Select start-up time | 1 (unprogrammed)(1) |
| SUT0 | 4 | Select start-up time | 0 (programmed)(1) |
| CKSEL3 | 3 | Select Clock source | 0 (programmed)(2) |
| CKSEL2 | 2 | Select Clock source | 0 (programmed)(2) |
| CKSEL1 | 1 | Select Clock source | 1 (unprogrammed)(2) |
| CKSEL0 | 0 | Select Clock source | 0 (programmed)(2) |
Note:
- The default value of SUT[1:0] results in maximum start-up time for the default clock source. See table Start-Up Times for the Internal Calibrated RC Oscillator Clock Selection - SUT in Calibrated Internal RC Oscillator of System Clock and Clock Options chapter for details.
- The default setting of CKSEL[3:0] results in internal RC Oscillator @ 8 MHz. See table Internal Calibrated RC Oscillator Operating Modes in Calibrated Internal RC Oscillator of the System Clock and Clock Options chapter for details.
- The CKOUT Fuse allows the system clock to be output on PORTB0. Refer to Clock Output Buffer section in the System Clock and Clock Options chapter for details.
- Refer to System Clock Prescaler section in the System Clock and Clock Options chapter for details.
The status of the Fuse bits is not affected by Chip Erase. Note that the Fuse bits are locked if Lock bit1 (LB1) is programmed. Program the Fuse bits before programming the Lock bits.