
27
8154B–AVR–07/09
ATmega16A
The CKSEL0 Fuse together with the SUT1:0 Fuses select the start-up times as shown in
TableNotes:
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.
8.5
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 8-2. 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 8-4.
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
(VCC = 5.0V)
Recommended Usage
0
00
4.1 ms
Ceramic resonator, fast rising
power
0
01
65 ms
Ceramic resonator, slowly
rising power
010
–
Ceramic resonator, BOD
enabled
011
4.1 ms
Ceramic resonator, fast rising
power
100
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