ATmega48/88/168
Table 27-7. Fuse Low Byte
Low Fuse Byte
CKDIV8(4)
CKOUT(3)
SUT1
Bit No
Description
Default Value
7
6
5
4
3
2
1
0
Divide clock by 8
Clock output
0 (programmed)
1 (unprogrammed)
1 (unprogrammed)(1)
0 (programmed)(1)
0 (programmed)(2)
0 (programmed)(2)
1 (unprogrammed)(2)
0 (programmed)(2)
Select start-up time
Select start-up time
Select Clock source
Select Clock source
Select Clock source
Select Clock source
SUT0
CKSEL3
CKSEL2
CKSEL1
CKSEL0
Note:
1. The default value of SUT1..0 results in maximum start-up time for the default clock source.
See Table 8-9 on page 35 for details.
2. The default setting of CKSEL3..0 results in internal RC Oscillator @ 8 MHz. See Table 8-8 on
page 35 for details.
3. The CKOUT Fuse allows the system clock to be output on PORTB0. See “Clock Output Buffer”
on page 36 for details.
4. See “System Clock Prescaler” on page 37 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.
27.2.1
Latching of Fuses
The fuse values are latched when the device enters programming mode and changes of the
fuse values will have no effect until the part leaves Programming mode. This does not apply to
the EESAVE Fuse which will take effect once it is programmed. The fuses are also latched on
Power-up in Normal mode.
27.3 Signature Bytes
All Atmel microcontrollers have a three-byte signature code which identifies the device. This
code can be read in both serial and parallel mode, also when the device is locked. The three
bytes reside in a separate address space. For the ATmega48/88/168 the signature bytes are
given in Table 27-8.
Table 27-8. Device ID
Signature Bytes Address
Part
0x000
0x1E
0x1E
0x1E
0x001
0x92
0x93
0x94
0x002
0x05
0x0A
0x06
ATmega48
ATmega88
ATmega168
27.4 Calibration Byte
The ATmega48/88/168 has a byte calibration value for the internal RC Oscillator. This byte
resides in the high byte of address 0x000 in the signature address space. During reset, this byte
is automatically written into the OSCCAL Register to ensure correct frequency of the calibrated
RC Oscillator.
289
2545M–AVR–09/07
ATMEL [ ATMEL ]
