ATmega640/1280/1281/2560/2561
Table 10-6. Start-up Times for the Full Swing Crystal Oscillator Clock Selection
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
Oscillator Source /
Power Conditions
(VCC = 5.0V)
CKSEL0
SUT1:0
Ceramic resonator, fast
rising power
258 CK
258 CK
1K CK
14CK + 4.1 ms(1)
14CK + 65 ms(1)
14CK(2)
0
00
Ceramic resonator, slowly
rising power
0
0
0
1
1
1
1
01
10
11
00
01
10
11
Ceramic resonator, BOD
enabled
Ceramic resonator, fast
rising power
1K CK
14CK + 4.1 ms(2)
14CK + 65 ms(2)
14CK
Ceramic resonator, slowly
rising power
1K CK
Crystal Oscillator, BOD
enabled
16K CK
16K CK
16K CK
Crystal Oscillator, fast
rising power
14CK + 4.1 ms
14CK + 65 ms
Crystal Oscillator, slowly
rising power
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.
10.6 Low Frequency Crystal Oscillator
The device can utilize a 32.768 kHz watch crystal as clock source by a dedicated Low Fre-
quency Crystal Oscillator. The crystal should be connected as shown in Figure 10-2. When this
Oscillator is selected, start-up times are determined by the SUT Fuses and CKSEL0 as shown in
Table 10-7.
Table 10-7. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
Power Conditions
BOD enabled
(VCC = 5.0V)
CKSEL0
SUT1:0
00
1K CK
1K CK
14CK(1)
0
0
0
0
1
1
1
1
Fast rising power
Slowly rising power
14CK + 4.1 ms(1)
14CK + 65 ms(1)
01
1K CK
10
Reserved
32K CK
32K CK
32K CK
Reserved
11
BOD enabled
Fast rising power
Slowly rising power
14CK
00
14CK + 4.1 ms
14CK + 65 ms
01
10
11
45
2549L–AVR–08/07
ATMEL [ ATMEL ]
