ATMEGA640 datasheet(42/436 Pages) MICROCHIP

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ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

Notes: 1. This is the recommended CKSEL settings for the different frequency ranges.

2. This option should not be used with crystals, only with ceramic resonators.

3. If 8MHz frequency exceeds the specification of the device (depends on VCC), the CKDIV8 Fuse can be pro-

grammed 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.

4. Maximum frequency when using ceramic oscillator is 10MHz.

The CKSEL0 Fuse together with the SUT1:0 Fuses select the start-up times as shown in Table 10-4.

Notes: 1. These options should only be used when not operating close to the maximum frequency of the device, and only if

frequency stability at start-up is not important for the application. These options are not suitable for crystals.

2. These options are intended for use with ceramic resonators and will ensure frequency stability at start-up. They can

also be used with crystals when not operating close to the maximum frequency of the device, and if frequency sta-

bility at start-up is not important for the application.

10.5 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 10-2 on page 41. 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 output. 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 41. 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 capacitance, and the electromagnetic noise of the environ-

ment. Some initial guidelines for choosing capacitors for use with crystals are given in Table 10-6 on page 43. 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 10-5 on page 43.

Table 10-3.

Low Power Crystal Oscillator Operating Modes(3)

Frequency Range [MHz]

CKSEL3:1(1)

Recommended Range for Capacitors C1 and C2 [pF]

0.4 - 0.9

100(2)

–

0.9 - 3.0

101

12 - 22

3.0 - 8.0

110

12 - 22

8.0 - 16.0(4)

111

12 - 22

Table 10-4.

Start-up Times for the Low Power Crystal Oscillator Clock Selection

Oscillator Source / Power Conditions

Start-up Time from Power-

down and Power-save

Additional Delay

from Reset

(VCC = 5.0V)

CKSEL0

SUT1:0

Ceramic resonator, fast rising power

258CK

14CK + 4.1ms(1)

000

Ceramic resonator, slowly rising power

258CK

14CK + 65ms(1)

001

Ceramic resonator, BOD enabled

1KCK

14CK(2)

010

Ceramic resonator, fast rising power

1KCK

14CK + 4.1ms(2)

011

Ceramic resonator, slowly rising power

1KCK

14CK + 65ms(2)

100

Crystal Oscillator, BOD enabled

16KCK

14CK

Crystal Oscillator, fast rising power

16KCK

14CK + 4.1ms

Crystal Oscillator, slowly rising power

16KCK

14CK + 65ms

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