Datasheet

30
2545T–AVR–05/11
ATmega48/88/168
Figure 9-2. Crystal oscillator connections.
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 9-3.
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 8MHz frequency exceeds the specification of the device (depends on V
CC
), the CKDIV8
Fuse can be programmed in order to divide the internal frequency by eight. 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
9-4.
Table 9-3. Low power crystal oscillator operating modes
(3)
.
Frequency range
(MHz)
Recommended range for
capacitors C1 and C2 (pF) CKSEL3..1
(1)
0.4 - 0.9 – 100
(2)
0.9 - 3.0 12 - 22 101
3.0 - 8.0 12 - 22 110
8.0 - 16.0 12 - 22 111
Table 9-4. Start-up times for the low power 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
258CK 14CK + 4.1ms
(1)
000
Ceramic resonator,
slowly rising power
258CK 14CK + 65ms
(1)
001
Ceramic resonator,
BOD enabled
1KCK 14CK
(2)
010
Ceramic resonator,
fast rising power
1KCK 14CK + 4.1ms
(2)
011
Ceramic resonator,
slowly rising power
1KCK 14CK + 65ms
(2)
100
XTAL2
XTAL1
GND
C2
C1