Datasheet

40
ATmega640/1280/1281/2560/2561
2549K–AVR–01/07
is dependent on the clock type, and varies from 6 cycles for an externally applied clock
to 32K cycles for a low frequency crystal.
The start-up sequence for the clock includes both the time-out delay and the start-up
time when the device starts up from reset. When starting up from Power-save or Power-
down mode, Vcc is assumed to be at a sufficient level and only the start-up time is
included.
Low Power Crystal
Oscillator
Pins XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier
which can be configured for use as an On-chip Oscillator, as shown in Figure 21. Either
a quartz crystal or a ceramic resonator may be used.
This Crystal Oscillator is a low power oscillator, with reduced voltage swing on the
XTAL2 output. It gives the lowest power consumption, but is not capable of driving other
clock inputs, and may be more susceptible to noise in noisy environments. In these
cases, refer to the “Full Swing Crystal Oscillator” on page 42.
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 capac-
itance, and the electromagnetic noise of the environment. Some initial guidelines for
choosing capacitors for use with crystals are given in Table 12. For ceramic resonators,
the capacitor values given by the manufacturer should be used.
Figure 21. Crystal Oscillator Connections
The Low Power Oscillator can operate in three different modes, each optimized for a
specific frequency range. The operating mode is selected by the fuses CKSEL3:1 as
shown in Table 12.
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.
Table 12. 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
(4)
111 12 - 22
XTAL2
XTAL1
GND
C2
C1