AT90PWM2 datasheet(32/365 Pages) ATMEL

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AT90PWM2/3

4317B–AVR–02/05

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 14. 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 capac-

itance, and the electromagnetic noise of the environment. Some initial guidelines for

choosing capacitors for use with crystals are given in Table 5. 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 14. Crystal Oscillator Connections

The Oscillator can operate in three different modes, each optimized for a specific fre-

quency range. The operating mode is selected by the fuses CKSEL3..1 as shown in

Table 5.

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.

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

in Table 6.

Table 5. Crystal Oscillator Operating Modes

CKSEL3..1

Frequency Range

(1) (MHz)

Recommended Range for Capacitors C1

and C2 for Use with Crystals (pF)

100

(2)

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

AT90PWM2 Datasheet(PDF) 32 Page - ATMEL Corporation

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