Datasheet

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
(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
010 1K CK
(2)
–
Ceramic resonator, BOD
enabled
011 1K CK
(2)
4.1 ms
Ceramic resonator, fast rising
power
100 1K CK
(2)
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
(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