Atmel ATmega128, Manual

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40
2467S–AVR–07/09
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 32.768 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 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 36 pF.
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)
4.1 ms Ceramic resonator, fast
rising power
0
01 258 CK
(1)
65 ms Ceramic resonator,
slowly rising power
0
10 1K CK
(2)
– Ceramic resonator,
BOD enabled
0
11 1K CK
(2)
4.1 ms Ceramic resonator, fast
rising power
1
00 1K CK
(2)
65 ms Ceramic resonator,
slowly rising power
1
01 16K CK – Crystal Oscillator, BOD
enabled
1
10 16K CK 4.1 ms Crystal Oscillator, fast
rising power
1
11 16K CK 65 ms 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)
4.1 ms Fast rising power or BOD enabled
01 1K CK
(1)
65 ms Slowly rising power
10 32K CK 65 ms Stable frequency at start-up
11 Reserved