33.2 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.

Table 33-5. Extended Fuse Byte for ATmega48PB
Extended Fuse ByteBit No.DescriptionDefault Value
71
61
51
41
31
21
11
SELFPRGEN0Self Programming Enable1 (unprogrammed)
Table 33-6. Extended Fuse Byte for ATmega88PB/168PB
Extended Fuse ByteBit No.DescriptionDefault Value
71
61
51
41
31
BOOTSZ12Select Boot Size (refer to Table Boot Size Configuration, ATmega88PB and Boot Size Configuration, ATmega168PB in Self-Programming the Flash for details)0 (programmed)(1)
BOOTSZ21Select Boot Size (refer to Table Boot Size Configuration, ATmega88PB and Boot Size Configuration, ATmega168PB in Self-Programming the Flash for details)0 (programmed)(1)
BOOTRST0Select Reset Vector1 (unprogrammed)

Note: 1. The default value of BOOTSZ[1:0] results in maximum Boot Size. See ”Pin Name Mapping”

Table 33-7. Fuse High Byte.
High Fuse ByteBit No.DescriptionDefault Value
RSTDISBL(1)7External Reset Disable1 (unprogrammed)
DWEN6debugWIRE Enable1 (unprogrammed)
SPIEN(2)5Enable Serial Program and Data Downloading0 (programmed, SPI programming enabled)
WDTON(3)4Watchdog Timer Always On1 (unprogrammed)
EESAVE3EEPROM memory is preserved through the Chip Erase1 (unprogrammed), EEPROM not reserved
BODLEVEL2(4)2Brown-out Detector trigger level1 (unprogrammed)
BODLEVEL1(4)1Brown-out Detector trigger level1 (unprogrammed)
BODLEVEL0(4)0Brown-out Detector trigger level1 (unprogrammed)
Note:
  1. Refer to Alternate Functions of Port C in I/O-Ports chapter for description of RSTDISBL Fuse.
  2. The SPIEN Fuse is not accessible in serial programming mode.
  3. Refer to WDTCSR – Watchdog Timer Control Register for details.
  4. Refer to Table BODLEVEL Fuse Coding in System and Reset Characteristics for BODLEVEL Fuse decoding.
Table 33-8. Fuse Low Byte
Low Fuse ByteBit No.DescriptionDefault Value
CKDIV8(4)7Divide clock by 80 (programmed)
CKOUT(3)6Clock output1 (unprogrammed)
SUT15Select start-up time1 (unprogrammed)(1)
SUT04Select start-up time0 (programmed)(1)
CKSEL33Select Clock source0 (programmed)(2)
CKSEL22Select Clock source0 (programmed)(2)
CKSEL11Select Clock source1 (unprogrammed)(2)
CKSEL00Select Clock source0 (programmed)(2)
Note:
  1. 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 in Calibrated Internal RC Oscillator of System Clock and Clock Options chapter for details.
  2. The default setting of CKSEL[3:0] results in internal RC Oscillator @ 8MHz. See Table 'Internal Calibrated RC Oscillator Operating Modes' in Calibrated Internal RC Oscillator of the System Clock and Clock Options chapter for details.
  3. 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.
  4. 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.