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AT90PWM216/316 [DATASHEET]
7710H–AVR–07/2013
6.3
Default Clock Source
The device is shipped with CKSEL = “0010”, SUT = “10”, and CKDIV8 programmed. The default clock source set-
ting is the Internal RC Oscillator with longest start-up time and an initial system clock prescaling of 8. This default
setting ensures that all users can make their desired clock source setting using an In-System or Parallel
programmer.
6.4
Low Power Crystal Oscillator
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 6-2. 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 low-
est power consumption, but is not capable of driving other clock inputs.
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 environ-
ment. 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. For more information on how to
choose capacitors and other details on Oscillator operation, refer to the Multi-purpose Oscillator Application Note.
Figure 6-2.
Crystal Oscillator Connections
The 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 6-3.Notes:
1. The frequency ranges are preliminary values.
2. This option should not be used with crystals, only with ceramic resonators.
The CKSEL0 Fuse together with the SUT1..0 Fuses select the start-up times as shown in
Table 6-4.
Table 6-3.
Crystal Oscillator Operating Modes
CKSEL3..1
Recommended Range for Capacitors C1 and
C2 for Use with Crystals (pF)
0.4 - 0.9
–
101
0.9 - 3.0
12 - 22
110
3.0 - 8.0
12 - 22
111
8.0 -16.0
12 - 22
XTAL2
XTAL1
GND
C2
C1