Datasheet
170
8197C–AVR–05/11
ATtiny261A/461A/861A
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 32 for details.
3. The default value results in maximum start-up time for the default clock source. See Table 6-7
on page 28 for details.
4. The default setting results in internal oscillator @ 8.0 MHz. See Table 6-6 on page 28 for
details.
Note that fuse bits are locked if Lock Bit 1 (LB1) is programmed. Fuse bits should be pro-
grammed before lock bits. The status of fuse bits is not affected by chip erase.
Fuse bits can also be read by device firmware. See section “Reading Fuse and Lock Bits from
Software” on page 165.
18.2.1 Latching of Fuses
Fuse values are latched when the device enters programming mode and changes to fuse values
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. Fuses are also latched on power-up.
18.3 Signature Bytes
All Atmel microcontrollers have a three-byte signature code which identifies the device. This
code can be read in both serial and High-voltage Programming mode, also when the device is
locked. The three bytes reside in a separate address space. The signature bytes are given in
Table 18-6.
18.4 Calibration Byte
The signature area has one byte of calibration data for the internal oscillator. This byte resides in
the high byte of address 0x000. During reset, this byte is automatically written into the OSCCAL
Register to ensure correct frequency of the calibrated oscillator.
Table 18-5. Fuse Low Byte
Fuse Low Byte Bit No Description Default Value
CKDIV8
(1)
7 Divide clock by 8 0 (programmed)
CKOUT
(2)
6 Clock Output Enable 1 (unprogrammed)
SUT1 5 Select start-up time 1 (unprogrammed)
(3)
SUT0 4 Select start-up time 0 (programmed)
(3)
CKSEL3 3 Select Clock source 0 (programmed)
(4)
CKSEL2 2 Select Clock source 0 (programmed)
(4)
CKSEL1 1 Select Clock source 1 (unprogrammed)
(4)
CKSEL0 0 Select Clock source 0 (programmed)
(4)
Table 18-6. Device ID
Parts
Signature Bytes Address
0x000 0x001 0x002
ATtiny261A 0x1E 0x91 0x0C
ATtiny461A 0x1E 0x92 0x08
ATtiny861A 0x1E 0x93 0x0D