Datasheet

Typical Characteristics – TA = -40°C to 85°C

13.3 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 13-2. Crystal Oscillator Connections

XTAL2

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 the following table.

Table 13-3. 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.

The CKSEL0 Fuse together with the SUT1:0 Fuses select the start-up times as shown in the next table.

AVR 8-Bit Microcontroller

System Clock and Clock Options

Datasheet Complete

40001974A-page 48