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2486AA–AVR–02/2013
ATmega8(L)
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 32.768kHz 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-
enable internal capacitors on XTAL1 and XTAL2, thereby removing the need for external capac-
itors. The internal capacitors have a nominal value of 36pF.
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Note:
1. These options should only be used if frequency stability at start-up is not important for the
application
External RC
Oscillator
can be used. The frequency is roughly estimated by the equation f = 1/(3RC). C should be at
Table 5. Start-up Times for the Crystal Oscillator Clock Selection
CKSEL0
SUT1..0
Start-up Time
from Power-down
and Power-save
Additional Delay
from Reset
(VCC = 5.0V)
Recommended Usage
0
00
4.1ms
Ceramic resonator, fast
rising power
0
01
65ms
Ceramic resonator, slowly
rising power
010
–
Ceramic resonator, BOD
enabled
011
4.1ms
Ceramic resonator, fast
rising power
100
65ms
Ceramic resonator, slowly
rising power
101
16K CK
–
Crystal Oscillator, BOD
enabled
1
10
16K CK
4.1ms
Crystal Oscillator, fast
rising power
1
11
16K CK
65ms
Crystal Oscillator, slowly
rising power
Table 6. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
SUT1..0
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(VCC = 5.0V)
Recommended Usage
00
4.1ms
Fast rising power or BOD enabled
01
65ms
Slowly rising power
10
32K CK
65ms
Stable frequency at start-up
11
Reserved