ATmega1284P datasheet(32/380 Pages) ATMEL

32 / 380 page

8059D–AVR–11/09

ATmega1284P

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.

ATmega1284P oscillator is optimized for very low power consumption, and thus when selecting

crystals, see Table 7-7 on page 32 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, see Table 7-8 on

page 32 at each TOSC pin.

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

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 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

CC = 5.0V)

CKSEL0

SUT1..0

Crystal CL (pF)

Max ESR [k

Ω](1)

9.0

12.5

Table 7-8.

Capacitance for Low-frequency Crystal Oscillator

Device

32kHz Osc. Type

Cap(Xtal1/Tosc1)

Cap(Xtal2/Tosc2)

ATmega1284P

System Osc.

18 pF

8 pF

Timer Osc.

6 pF

2 CL

⋅

–

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