Datasheet

30
7766F–AVR–11/10
ATmega16/32U4
C1 and C2 should always be equal for both crystals and resonators. The optimal value of the
capacitors depends on the crystal or resonator in use, the amount of stray capacitance, and the
electromagnetic noise of the environment. Some initial guidelines for choosing capacitors for
use with crystals are given in Table 6-3. For ceramic resonators, the capacitor values given by
the manufacturer should be used.
Figure 6-2. 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 CKSEL[3..1] as shown in Table 6-3.
Notes: 1. The frequency ranges are preliminary values. Actual values are TBD.
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 V
CC
), 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
6-4.
Table 6-3. Low Power Crystal Oscillator Operating Modes
(3)
Frequency Range
(1)
(MHz) CKSEL3..1
Recommended Range for Capacitors
C1 and C2 (pF)
0.4 - 0.9 100
(2)
–
0.9 - 3.0 101 12 - 22
3.0 - 8.0 110 12 - 22
8.0 - 16.0 111 12 - 22
Table 6-4. 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
258 CK 14CK + 4.1 ms
(1)
000
Ceramic resonator,
slowly rising power
258 CK 14CK + 65 ms
(1)
001
Ceramic resonator,
BOD enabled
1K CK 14CK
(2)
010
Ceramic resonator, fast
rising power
1K CK 14CK + 4.1 ms
(2)
011
XTAL2
XTAL1
GND
C2
C1