ATMEGA640 datasheet(42/449 Pages) ATMEL

42 / 449 page

ATmega640/1280/1281/2560/2561

2549K–AVR–01/07

Full Swing Crystal

Oscillator

Pins 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 21. Either

a quartz crystal or a ceramic resonator may be used.

This Crystal Oscillator is a full swing oscillator, with rail-to-rail swing on the XTAL2 out-

put. This is useful for driving other clock inputs and in noisy environments. The current

consumption is higher than the “Low Power Crystal Oscillator” on page 40. Note that the

Full Swing Crystal Oscillator will only operate for Vcc = 2.7 - 5.5 volts.

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 15. For ceramic resonators,

the capacitor values given by the manufacturer should be used.

The operating mode is selected by the fuses CKSEL3:1 as shown in Table 14.

Notes:

1. The frequency ranges are preliminary values. Actual values are TBD.

2. If 8 MHz frequency exceeds the specification of the device (depends on VCC), the

CKDIV8 Fuse can be programmed in order to divide the internal frequency by 8. It

must be ensured that the resulting divided clock meets the frequency specification of

the device.

Figure 22. Crystal Oscillator Connections

Table 14. Full Swing Crystal Oscillator operating modes

(2)

Frequency Range(1) (MHz)

CKSEL3:1

Recommended Range for Capacitors

C1 and C2 (pF)

0.4 - 16

011

12 - 22

XTAL2

XTAL1

GND

ATMEGA640 Datasheet(PDF) 42 Page - ATMEL Corporation