Datasheet
29
7728G–AVR–06/10
ATtiny87/ATtiny167
The CKSEL0 Fuse together with the SUT1..0 Fuses or CSEL0 together with CSUT1..0 field
select the start-up times as shown in Table 4-7.
Notes: 1. Flash Fuse bits.
2. CLKSELR register bits.
3. These options should only be used when not operating close to the maximum frequency of
the device, and only if frequency stability at start-up is not important for the application.
These options are not suitable for crystals.
4. These options are intended for use with ceramic resonators and will ensure frequency stabil-
ity at start-up. They can also be used with crystals when not operating close to the maximum
frequency of the device, and if frequency stability at start-up is not important for the
application.
5. This setting is only available if RSTDISBL fuse is not set.
4.2.5 Low-frequency Crystal Oscillator
To use a 32.768 kHz watch crystal as the clock source for the device, the low-frequency crys-
tal oscillator must be selected by setting CKSEL fuses or CSEL field as shown in Table 4-1 on
page 25. The crystal should be connected as shown in Figure 4-3. Refer to the 32.768 kHz
Crystal Oscillator Application Note for details on oscillator operation and how to choose appro-
priate values for C1 and C2.
The 32.768 kHz watch crystal oscillator can be used by the asynchronous timer if the
(high-frequency) Crystal Oscillator is not running or if the External Clock is not enabed (See
”Enable/Disable Clock Source” on page 32.). The asynchronous timer is then able to start
itself this low-frequency crystal oscillator.
Table 4-7. Start-up Times for the Crystal Oscillator Clock Selection
CKSEL0
(1)
CSEL0
(2)
SUT1..0
(1)
CSUT1..0
(2)
Start-up Time from
Power-down/save
Additional Delay
from Reset
(Vcc = 5.0V) Recommended Usage
0 00 258 CK
(3)
14CK + 4.1 ms
Ceramic resonator, fast
rising power
0 01 258 CK
(3)
14CK + 65 ms
Ceramic resonator, slowly
rising power
010
(5)
1K (1024) CK
(4)
14CK
Ceramic resonator, BOD
enabled
0 11 1K (1024)CK
(4)
14CK + 4.1 ms
Ceramic resonator, fast
rising power
1 00 1K (1024)CK
(4)
14CK + 65 ms
Ceramic resonator, slowly
rising power
101
(5)
16K (16384) CK 14CK
Crystal Oscillator, BOD
enabled
1 10 16K (16384) CK 14CK + 4.1 ms
Crystal Oscillator, fast
rising power
1 11 16K (16384) CK 14CK + 65 ms
Crystal Oscillator, slowly
rising power