Datasheet
149
ATtiny25/45/85 [DATASHEET]
2586Q–AVR–08/2013
Notes: 1. See “System Clock Prescaler” on page 31 for details.
2. Allows system clock to be output on pin. See “Clock Output Buffer” on page 31 for details.
3. The default value gives maximum start-up time for the default clock source. See Table 6-7 on page 28 for details.
4. The default setting selects internal, 8 MHz RC oscillator. See Table 6-6 on page 27 for details.
Note that fuse bits are locked if Lock Bit 1 (LB1) is programmed. Fuse bits should be programmed before lock bits.
The status of fuse bits is not affected by chip erase.
Lock bits can also be read by device firmware. See section “Reading Lock, Fuse and Signature Data from Soft-
ware” on page 143.
20.2.1 Latching of Fuses
Fuse values are latched when the device enters programming mode and changes to fuse values will have no effect
until the part leaves programming mode. This does not apply to the EESAVE Fuse which takes effect once it is pro-
grammed. Fuses are also latched on power-up.
20.3 Device Signature Imprint Table
The device signature imprint table is a dedicated memory area used for storing miscellaneous device information,
such as the device signature and oscillator calibration data. Most of this memory segment is reserved for internal
use, as outlined in Table 20-6.
Notes: 1. See section “Signature Bytes” for more information.
2. See section “Calibration Bytes” for more information.
Table 20-5. Fuse Low Byte
Fuse Low Byte Bit No Description Default Value
CKDIV8
(1)
7 Clock divided by 8 0 (programmed)
CKOUT
(2)
6 Clock output enabled 1 (unprogrammed)
SUT1
(3)
5 Start-up time setting 1 (unprogrammed)
(3)
SUT0
(3)
4 Start-up time setting 0 (programmed)
(3)
CKSEL3
(4)
3 Clock source setting 0 (programmed)
(4)
CKSEL2
(4)
2 Clock source setting 0 (programmed)
(4)
CKSEL1
(4)
1 Clock source setting 1 (unprogrammed)
(4)
CKSEL0
(4)
0 Clock source setting 0 (programmed)
(4)
Table 20-6. Contents of Device Signature Imprint Table.
Address High Byte
0x00 Signature byte 0
(1)
0x01 Calibration data for internal oscillator at 8.0 MHz
(2)
0x02 Signature byte 1
(1)
0x03 Calibration data for internal oscillator at 6.4 MHz
(2)
0x04 Signature byte 2
(1)
0x05 ... 0x2A Reserved for internal use