ATMEGA640 datasheet(44/436 Pages) MICROCHIP

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

DS40002211A

The capacitance (Ce + Ci) needed at each XTAL/TOSC pin can be calculated by using:

Ce + Ci = 2 * CL - Cs

where:

Ce - is optional external capacitors as described in Figure 10-3.

Ci - is the pin capacitance in Table 10-7 on page 43.

CL - is the load capacitance for a 32.768kHz crystal specified by the crystal vendor.

CS - is the total stray capacitance for one XTAL/TOSC pin.

Crystals specifying load capacitance (CL) higher than the ones given in the Table 10-7 on page 43, require exter-

nal capacitors applied as described in Figure 10-3.

Figure 10-3. Crystal Oscillator Connections

To find suitable load capacitance for a 32.768kHz crystal, consult the crystal datasheet.

When this oscillator is selected, start-up times are determined by the SUT Fuses and CKSEL0 as shown in Table

10-8.

Note:

1. These options should only be used if frequency stability at start-up is not important for the application.

Table 10-8.

Start-up times for the low frequency crystal oscillator clock selection

Power Conditions

Start-up Time from Power-down

and Power-save

Additional Delay from Reset

(VCC = 5.0V)

CKSEL0

SUT1:0

BOD enabled

1K CK

14CK(1)

000

Fast rising power

1K CK

14CK + 4.1ms(1)

001

Slowly rising power

1K CK

14CK + 65ms(1)

010

Reserved

BOD enabled

32K CK

14CK

Fast rising power

32K CK

14CK + 4.1ms

Slowly rising power

32K CK

14CK + 65ms

Reserved

TOSC2

TOSC1

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