Table 5. Start-up Times for the Crystal Oscillator Clock Selection
Start-up Time
from Power-down
and Power-save
Additional Delay
from Reset
CKSEL0 SUT1..0
(VCC = 5.0V)
Recommended Usage
Ceramic resonator, fast
rising power
0
0
0
0
1
1
1
1
00
01
10
11
00
01
10
11
258 CK(1)
258 CK(1)
1K CK(2)
1K CK(2)
1K CK(2)
16K CK
16K CK
16K CK
4.1 ms
65 ms
–
Ceramic resonator, slowly
rising power
Ceramic resonator, BOD
enabled
Ceramic resonator, fast
rising power
4.1 ms
65 ms
–
Ceramic resonator, slowly
rising power
Crystal Oscillator, BOD
enabled
Crystal Oscillator, fast
rising power
4.1 ms
Crystal Oscillator, slowly
rising power
65 ms
Notes: 1. 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.
2. These options are intended for use with ceramic resonators and will ensure frequency stability
at start-up. They can also be used with crystals when not operating close to the maximum fre-
quency of the device, and if frequency stability at start-up is not important for the application.
Low-frequency
Crystal Oscillator
To use a 32.768 kHz watch crystal as the clock source for the device, the Low-frequency Crystal
Oscillator must be selected by setting the CKSEL Fuses to “1001”. The crystal should be con-
nected as shown in Figure 11. By programming the CKOPT Fuse, the user can enable internal
capacitors on XTAL1 and XTAL2, thereby removing the need for external capacitors. The inter-
nal capacitors have a nominal value of 36 pF.
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Table 6.
Table 6. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
SUT1..0
00
(VCC = 5.0V)
Recommended Usage
1K CK(1)
1K CK(1)
32K CK
4.1 ms
65 ms
65 ms
Fast rising power or BOD enabled
Slowly rising power
01
10
Stable frequency at start-up
11
Reserved
Note:
1. These options should only be used if frequency stability at start-up is not important for the
application.
External RC
Oscillator
For timing insensitive applications, the external RC configuration shown in Figure 12 can be
used. The frequency is roughly estimated by the equation f = 1/(3RC). C should be at least 22
28
ATmega8(L)
2486T–AVR–05/08
ATMEL [ ATMEL ]
