ATMEGA8_08 Datasheet, PDF (28/308 Pages) ATMEL Corporation – 8-bit with 8K Bytes In-System Programmable Flash

Table 5. Start-up Times for the Crystal Oscillator Clock Selection
CKSEL0 SUT1..0
0
00
Start-up Time
from Power-down
and Power-save
258 CK⁽¹⁾
Additional Delay
from Reset
(V_CC = 5.0V)
4.1 ms
Recommended Usage
Ceramic resonator, fast
rising power
0
01
258 CK⁽¹⁾
65 ms
Ceramic resonator, slowly
rising power
0
10
1K CK⁽²⁾
–
Ceramic resonator, BOD
enabled
0
11
1K CK⁽²⁾
4.1 ms
Ceramic resonator, fast
rising power
1
00
1K CK⁽²⁾
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
Notes:
11
16K CK
65 ms
Crystal Oscillator, slowly
rising power
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.
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 11. 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 6.
Table 6. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
SUT1..0
00
Start-up Time from
Power-down and
Power-save
1K CK⁽¹⁾
Additional Delay
from Reset
(V_CC = 5.0V)
4.1 ms
Recommended Usage
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
Note: 1. These options should only be used if frequency stability at start-up is not important for the
application.
External RC
Oscillator
For timing insensitive applications, the external RC configuration shown in Figure 12 can be
used. The frequency is roughly estimated by the equation f = 1/(3RC). C should be at least 22
28 ATmega8(L)
2486T–AVR–05/08