ATMEGA640 datasheet(43/436 Pages) MICROCHIP

43 / 436 page

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

DS40002211A

Note:

1. 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.

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.

10.6 Low Frequency Crystal Oscillator

The device can utilize a 32.768kHz watch crystal as clock source by a dedicated Low Frequency Crystal Oscillator.

The crystal should be connected as shown in Figure 10-3 on page 44. When this Oscillator is selected, start-up

times are determined by the SUT Fuses and CKSEL0 as shown in Table 10-8 on page 44.

The Low-Frequency Crystal Oscillator provides an internal load capacitance, see Table 10-7 at each XTAL/TOSC

pin.

Table 10-5.

Full Swing Crystal Oscillator operating modes(1)

Frequency Range [MHz]

CKSEL3:1

Recommended Range for Capacitors C1 and C2 [pF]

0.4 - 16

011

12 - 22

Table 10-6.

Start-up Times for the Full Swing 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

258 CK

14CK + 4.1ms(1)

000

Ceramic resonator,

slowly rising power

258 CK

14CK + 65ms(1)

001

Ceramic resonator,

BOD enabled

1K CK

14CK(2)

010

Ceramic resonator,

fast rising power

1K CK

14CK + 4.1ms(2)

011

Ceramic resonator,

slowly rising power

1K CK

14CK + 65ms(2)

100

Crystal Oscillator,

BOD enabled

16K CK

14CK

Crystal Oscillator,

fast rising power

16K CK

14CK + 4.1ms

Crystal Oscillator,

slowly rising power

16K CK

14CK + 65ms

Table 10-7.

Capacitance for Low frequency oscillator

Device

32kHz oscillator

Cap (Xtal1/Tosc1)

Cap (Xtal2/Tosc2)

ATmega640/1280/1281/2560/2561

System Osc.

18pF

8pF

Timer Osc.

6pF

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