ATmega328P datasheet(31/345 Pages) ATMEL

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7810A–AVR–11/09

ATmega328P [Preliminary]

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. ATmega328P

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

Table 8-7 on page 31 for maximum ESR recommendations on 6.5 pF, 9.0 pF and 12.5 pF

crystals

Table 8-7.

Maximum ESR Recommendation for 32.768 kHz Crystal

Note:

1. Maximum ESR is typical value based on characterization

The Low-frequency Crystal Oscillator provides an internal load capacitance of typical 6 pF at

each TOSC pin. The external capacitance (C) needed at each TOSC pin can be calculated by

using:

where CL is the load capacitance for a 32.768 kHz crystal specified by the crystal vendor and C

is the total stray capacitance for one TOSC pin.

Crystals specifying load capacitance (CL) higher than 6 pF, require external capacitors applied

as described in Figure 8-2 on page 28.

The Low-frequency Crystal Oscillator must be selected by setting the CKSEL Fuses to “0110” or

“0111”, as shown in Table 8-9. Start-up times are determined by the SUT Fuses as shown in

Table 8-8.

Note:

1. This option should only be used if frequency stability at start-up is not important for the

application

Crystal CL (pF)

Max ESR [k

Ω](1)

6.5

9.0

12.5

Table 8-8.

Start-up Times for the Low-frequency Crystal Oscillator Clock Selection

SUT1..0

Additional Delay from Reset (VCC = 5.0V) Recommended Usage

4 CK

Fast rising power or BOD enabled

4 CK + 4.1 ms

Slowly rising power

4 CK + 65 ms

Stable frequency at start-up

Reserved

Table 8-9.

Start-up Times for the Low-frequency Crystal Oscillator Clock Selection

CKSEL3..0

Start-up Time from

Power-down and Power-save

Recommended Usage

0100(1)

1K CK

0101

32K CK

Stable frequency at start-up

–

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