Datasheet

39
2467X–AVR–06/11
ATmega128
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.
When this Oscillator is selected, start-up times are determined by the SUT fuses as shown in
Table 10.
Note: 1. These options should only be used if frequency stability at start-up is not important for the
application.
Table 9. 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
(V
CC
= 5.0V) Recommended Usage
0
00 258 CK
(1)
4.1ms Ceramic resonator, fast
rising power
0
01 258 CK
(1)
65ms Ceramic resonator,
slowly rising power
0
10 1K CK
(2)
– Ceramic resonator,
BOD enabled
0
11 1K CK
(2)
4.0ms Ceramic resonator, fast
rising power
1
00 1K CK
(2)
65ms Ceramic resonator,
slowly rising power
1
01 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 10. 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
(V
CC
= 5.0V) Recommended Usage
00 1K CK
(1)
4.1ms Fast rising power or BOD enabled
01 1K CK
(1)
65ms Slowly rising power
10 32K CK 65ms Stable frequency at start-up
11 Reserved