ATMEGA8_14 Datasheet, PDF (28/331 Pages) ATMEL Corporation – High-performance, Low-power Atmel

ATmega8(L)
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)
4.1ms
Recommended Usage
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
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 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 32.768kHz 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 con-
nected as shown in Figure 11 on page 27. By programming the CKOPT Fuse, the user can
enable internal capacitors on XTAL1 and XTAL2, thereby removing the need for external capac-
itors. 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
Table 6.
Table 6. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
SUT1..0
00
01
Start-up Time from
Power-down and
Power-save
1K CK⁽¹⁾
1K CK⁽¹⁾
Additional Delay
from Reset
(V_CC = 5.0V)
4.1ms
65ms
Recommended Usage
Fast rising power or BOD enabled
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
External RC
Oscillator
For timing insensitive applications, the external RC configuration shown in Figure 12 on page 29
can be used. The frequency is roughly estimated by the equation f = 1/(3RC). C should be at
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