ATmega1284P datasheet(35/468 Pages) ATMEL

35 / 468 page

8152G–AVR–11/09

ATmega164PA/324PA/644PA/1284P

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.

8.5

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.

ATmega164PA/324PA/644PA/1284P oscillator is optimized for very low power consumption,

and thus when selecting crystals, see Table 8-7 on page 35 for maximum ESR recommenda-

tions on 9 pF and 12.5 pF crystals.

Note:

1. Maximum ESR is typical value based on characterization

The Low-frequency Crystal Oscillator provides an internal load capacitance, see Table on page

35 at each TOSC pin.

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

where:

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

Table 8-6.

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

CC = 5.0V)

CKSEL0

SUT1..0

Table 8-7.

Maximum ESR Recommendation for 32.768 kHz Watch Crystal

Crystal CL (pF)

Max ESR [k

Ω](1)

9.0

12.5

Table 8-8.

Capacitance for Low-frequency Oscillator

Device

32kHz Osc. Type

Cap (Xtal1/Tosc1)

Cap (Xtal2/Tosc2)

ATmega164PA/324PA/644PA/1284P

System Osc.

18 pF

8 pF

Timer Osc.

6 pF

2 CL

⋅

–

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