ATmega48/88/168
Figure 8-2. Crystal Oscillator Connections
C2
XTAL2
C1
XTAL1
GND
The Low Power Oscillator can operate in three different modes, each optimized for a specific fre-
quency range. The operating mode is selected by the fuses CKSEL3..1 as shown in Table 8-3
on page 31.
Table 8-3.
Low Power Crystal Oscillator Operating Modes(3)
Frequency Range
Recommended Range for
Capacitors C1 and C2 (pF)
(MHz)
CKSEL3..1(1)
100(2)
101
0.4 - 0.9
0.9 - 3.0
3.0 - 8.0
8.0 - 16.0
–
12 - 22
12 - 22
12 - 22
110
111
Notes: 1. This is the recommended CKSEL settings for the different frequency ranges.
2. This option should not be used with crystals, only with ceramic resonators.
3. If 8 MHz frequency exceeds the specification of the device (depends on VCC), the CKDIV8
Fuse can be programmed 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.
The CKSEL0 Fuse together with the SUT1..0 Fuses select the start-up times as shown in Table
8-4.
Table 8-4.
Start-up Times for the Low Power Crystal Oscillator Clock Selection
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
Oscillator Source /
Power Conditions
(VCC = 5.0V)
CKSEL0
SUT1..0
Ceramic resonator, fast
rising power
258 CK
258 CK
1K CK
1K CK
1K CK
14CK + 4.1 ms(1)
14CK + 65 ms(1)
14CK(2)
0
00
Ceramic resonator, slowly
rising power
0
0
0
1
01
10
11
00
Ceramic resonator, BOD
enabled
Ceramic resonator, fast
rising power
14CK + 4.1 ms(2)
14CK + 65 ms(2)
Ceramic resonator, slowly
rising power
31
2545M–AVR–09/07
ATMEL [ ATMEL ]
