ATmega128
Table 9. Start-up Times for the Crystal Oscillator Clock Selection
Start-up Time from
Power-down and
Additional Delay
from Reset
258 CK
CKSEL0
0
0
0
0
1
1
1
1
SUT1..0
00
01
10
11
00
01
10
11
Power-save
(1)
258 CK (1)
1K CK (2)
1K CK (2)
1K CK (2)
16K CK
16K CK
16K CK
(V CC = 5.0V)
4.1ms
65ms
4.0ms
65ms
4.1ms
65ms
Recommended Usage
Ceramic resonator, fast
rising power
Ceramic resonator,
slowly rising power
Ceramic resonator,
BOD enabled
Ceramic resonator, fast
rising power
Ceramic resonator,
slowly rising power
Crystal Oscillator, BOD
enabled
Crystal Oscillator, fast
rising power
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.
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-
nected as shown in Figure 19 . 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 36pF.
W hen this Oscillator is selected, start-up times are determined by the SUT fuses as shown in
Table 10 .
Table 10. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
Start-up Time from
Power-down and
Additional Delay
from Reset
1K CK
1K CK
SUT1..0
00
01
10
Power-save
(1)
(1)
32K CK
(V CC = 5.0V)
4.1ms
65ms
65ms
Recommended Usage
Fast rising power or BOD enabled
Slowly rising power
Stable frequency at start-up
11
Note:
Reserved
1. These options should only be used if frequency stability at start-up is not important for the
application.
39
2467X–AVR–06/11
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