5.3
5.4
Default Clock Source
The device is shipped with CKSEL = “0010”, SUT = “10”, and CKDIV8 programmed. The default clock source setting 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.
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 5-2. 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 lowest 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 environment. Some initial
guidelines for choosing capacitors for use with crystals are given in Table 5-3. 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 5-2. Crystal Oscillator Connections
C2
XTAL2
C1
XTAL1
GND
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 5-3.
Table 5-3. Crystal Oscillator Operating Modes
Recommended Range for Capacitors C1 and C2 for
CKSEL3..1
100(1)
101
Frequency Range (MHz)
Use with Crystals (pF)
0.4 - 0.9
–
0.9 - 3.0
12 - 22
12 - 22
12 - 22
110
3.0 - 8.0
111
8.0 -16.0
Note:
1. This option should not be used with crystals, only with ceramic resonators.
ATmega16/32/64/M1/C1 [DATASHEET]
27
7647O–AVR–01/15
MICROCHIP [ MICROCHIP ]
