29.2 Fuse Bits
The ATmega8A has two 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.
| High Fuse Byte | Bit No. | Description | Default Value |
|---|---|---|---|
| RSTDISBL(4) | 7 | Select if PC6 is I/O pin or RESET pin | 1 (unprogrammed, PC6 is RESET-pin) |
| WDTON | 6 | WDT always on | 1 (unprogrammed, WDT enabled by WDTCR) |
| SPIEN(1) | 5 | Enable Serial Program and Data Downloading | 0 (programmed, SPI prog. enabled) |
| CKOPT(2) | 4 | Oscillator options | 1 (unprogrammed) |
| EESAVE | 3 | EEPROM memory is preserved through the Chip Erase | 1 (unprogrammed), EEPROM not reserved |
| BOOTSZ1 | 2 | Select Boot Size (see ATmega8A Boot Loader Parameters) | 0 (programmed)(3) |
| BOOTSZ0 | 1 | Select Boot Size (see ATmega8A Boot Loader Parameters) | 0 (programmed)(3) |
| BOOTRST | 0 | Select Reset Vector | 1 (unprogrammed) |
Note:
- The SPIEN Fuse is not accessible in Serial Programming mode.
- The CKOPT Fuse functionality depends on the setting of the CKSEL bits, see Clock Sources for details.
- The default value of BOOTSZ1:0 results in maximum Boot Size. See Boot Loader Parameters for details.
- When programming the RSTDISBL Fuse Parallel Programming has to be used to change fuses or perform further programming.
| Low Fuse Byte | Bit No. | Description | Default Value |
|---|---|---|---|
| BODLEVEL | 7 | Brown out detector trigger level | 1 (unprogrammed) |
| BODEN | 6 | Brown out detector enable | 1 (unprogrammed, BOD disabled) |
| 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 | 0 (programmed)(2) |
| CKSEL0 | 0 | Select Clock source | 1 (unprogrammed)(2) |
Note:
- The default value of SUT1:0 results in maximum start-up time. See table Start-up Times for the Internal Calibrated RC Oscillator Clock Selection in Calibrated Internal RC Oscillator of the System Clock and Clock Options chapter for details.
- The default setting of CKSEL3:0 results in internal RC Oscillator @ 1MHz. See table Internal Calibrated RC Oscillator Operating Modes in Calibrated Internal RC Oscillator of 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.