Datasheet

The CKSEL0 Fuse together with the SUT1:0 Fuses select the start-up times as shown in the next table.
Table 13-4 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)
4.1ms 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
Note:
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 frequency
of the device, and if frequency stability at start-up is not important for the application.
13.5. 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 connected as shown in
Figure 13-2 Crystal Oscillator Connections on page 54. By programming the CKOPT Fuse, the user can
enable internal capacitors on XTAL1 and XTAL2, thereby removing the need for external capacitors. 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 the table
below.
Table 13-5 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
Note: 1. These options should only be used if frequency stability at start-up is not important for the
application.
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
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