Datasheet

The CKSEL0 Fuse together with the SUT1:0 Fuses select the start-up times as shown in the next table.
Table 12-4. 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.
12.5. 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 12-2. 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 12-5. 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 ATmega32A [DATASHEET]
Atmel-8155I-ATmega32A_Datasheet_Complete-08/2016
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