Datasheet

12.3. Default Clock Source
The device is shipped with CKSEL = “0001” and SUT = “10”. The default clock source setting is therefore
the Internal RC Oscillator with longest startup time. This default setting ensures that all users can make
their desired clock source setting using an In-System or Parallel Programmer.
12.4. 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 the figure below. Either a quartz crystal or a ceramic resonator
may be used. The CKOPT Fuse selects between two different Oscillator amplifier modes. When CKOPT
is programmed, the Oscillator output will oscillate a full rail-to-rail swing on the output. This mode is
suitable when operating in a very noisy environment or when the output from XTAL2 drives a second
clock buffer. This mode has a wide frequency range. When CKOPT is unprogrammed, the Oscillator has
a smaller output swing. This reduces power consumption considerably. This mode has a limited
frequency range and it cannot be used to drive other clock buffers.
For resonators, the maximum frequency is 8MHz with CKOPT unprogrammed and 16MHz with CKOPT
programmed. 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 the next table. For ceramic resonators, the capacitor values given by the
manufacturer should be used.
Figure 12-2. Crystal Oscillator Connections
XTAL2
XTAL1
GND
C2
C1
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 the following table.
Table 12-3. Crystal Oscillator Operating Modes
CKOPT
(1)
CKSEL3:1 Frequency Range(MHz) Recommended Range for Capacitors C1 and C2
for Use with Crystals (pF)
1 101
(2)
0.4 - 0.9 –
1 110 0.9 - 3.0 12 - 22
1 111 3.0 - 8.0 12 - 22
0 101, 110, 111 1.0 -16.0 12 - 22
Note:
1. When CKOPT is programmed (0), the oscillator output will be a full rail-to-rail swing on the output.
2. This option should not be used with crystals, only with ceramic resonators.
Atmel ATmega32A [DATASHEET]
Atmel-8155I-ATmega32A_Datasheet_Complete-08/2016
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