27
ATmega8A [DATASHEET]
8159E–AVR–02/2013
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 frequency of the device, and if frequency sta-
bility at start-up is not important for the application.
9.4
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 connected as shown in
Figure 9-2. By pro-
gramming 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 36 pF.
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Table 9-5.
Note:
1. These options should only be used if frequency stability at start-up is not important for the application.
9.5
External RC Oscillator
For timing insensitive applications, the external RC configuration shown in
Figure 9-3 can be used. The frequency
is roughly estimated by the equation f = 1/(3RC). C should be at least 22 pF. By programming the CKOPT Fuse,
the user can enable an internal 36 pF capacitor between XTAL1 and GND, thereby removing the need for an exter-
nal capacitor.
Table 9-4.
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
101
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 9-5.
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
(VCC = 5.0V)
Recommended Usage
00
4.1 ms
Fast rising power or BOD enabled
01
65 ms
Slowly rising power
10
32K CK
65 ms
Stable frequency at start-up
11
Reserved