User Manual

30
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
8271E–AVR–07/2012
The operating mode is selected by the fuses CKSEL3...1 as shown in Table 9-5.
Notes: 1. If the cryatal frequency exceeds the specification of the device (depends on V
CC
), the CKDIV8 Fuse can be pro-
grammed in order to divide the internal frequency by 8. It must be ensured that the resulting divided clock meets
the frequency specification of the device.
Figure 9-3. Crystal Oscillator Connections
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.
Table 9-5. Full Swing Crystal Oscillator operating modes
Frequency Range
(1)
(MHz)
Recommended Range for
Capacitors C1 and C2 (pF) CKSEL3...1
0.4 - 20 12 - 22 011
Table 9-6. Start-up Times for the Full Swing Crystal Oscillator Clock Selection
Oscillator Source /
Power Conditions
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(V
CC
= 5.0V) CKSEL0 SUT1...0
Ceramic resonator, fast
rising power
258 CK 14CK + 4.1ms
(1)
000
Ceramic resonator, slowly
rising power
258 CK 14CK + 65ms
(1)
001
Ceramic resonator, BOD
enabled
1K CK 14CK
(2)
010
Ceramic resonator, fast
rising power
1K CK 14CK + 4.1ms
(2)
011
Ceramic resonator, slowly
rising power
1K CK 14CK + 65ms
(2)
100
Crystal Oscillator, BOD
enabled
16K CK 14CK 1 01
Crystal Oscillator, fast
rising power
16K CK 14CK + 4.1ms 1 10
Crystal Oscillator, slowly
rising power
16K CK 14CK + 65ms 1 11
XTAL2 (TOSC2)
XTAL1 (TOSC1)
GND
C2
C1