ATmega324P datasheet(34/377 Pages) ATMEL

34 / 377 page

7674F–AVR–09/09

ATmega164P/324P/644P

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 fre-

quency of the device, and if frequency stability at start-up is not important for the application.

7.5

Low Frequency Crystal Oscillator

The Low-frequency Crystal Oscillator is optimized for use with a 32.768 kHz watch crystal.

When selecting crystals, load capasitance and crystal’s Equivalent Series Resistance, ESR

must be taken into consideration. Both values are specified by the crystal vendor.

ATmega164P/324P/644P oscillator is optimized for very low power consumption, and thus when

selecting crystals, see Table 7-7 on page 34 for maximum ESR recommendations on 9 pF and

12.5 pF crystals

Table 7-7.

Maximum ESR Recommendation for 32.768 kHz Watch Crystal

Note:

1. Maximum ESR is typical value based on characterization

The Low-frequency Crystal Oscillator provides an internal load capacitance of typical 8.0 pF.

Crystals with recommended 8.0 pF load capacitance can be without external capacitors as

shown in Figure 7-3 on page 35.

Table 7-6.

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 + 4.1 ms(1)

000

Ceramic resonator, slowly

rising power

258 CK

14CK + 65 ms(1)

001

Ceramic resonator, BOD

enabled

1K CK

14CK(2)

010

Ceramic resonator, fast

rising power

1K CK

14CK + 4.1 ms(2)

011

Ceramic resonator, slowly

rising power

1K CK

14CK + 65 ms(2)

100

Crystal Oscillator, BOD

enabled

16K CK

14CK

Crystal Oscillator, fast

rising power

16K CK

14CK + 4.1 ms

Crystal Oscillator, slowly

rising power

16K CK

14CK + 65 ms

Crystal CL (pF)

Max ESR [k

Ω](1)

9.0

12.5

ATmega324P Datasheet(PDF) 34 Page - ATMEL Corporation

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