ATmega325P datasheet(32/364 Pages) ATMEL

32 / 364 page

8023F–AVR–07/09

ATmega325P/3250P

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.

ATmega325P/3250P oscillator is optimized for very low power consumption, and thus when

selecting crystals, see Table 8-5 on page 32 for maximum ESR recommendations on 6.5 pF,

9.0 pF and 12.5 pF crystals

Table 8-5.

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 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 31.

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

011

1K CK(2)

14CK + 4.1 ms

Ceramic resonator, fast

rising power

100

1K CK(2)

14CK + 65 ms

Ceramic resonator,

slowly rising power

16K CK

14CK

Crystal Oscillator, BOD

enabled

16K CK

14CK + 4.1 ms

Crystal Oscillator, fast

rising power

16K CK

14CK + 65 ms

Crystal Oscillator, slowly

rising power

Table 8-4.

Start-up Times for the Crystal Oscillator Clock Selection (Continued)

CKSEL0

SUT1..0

Start-up Time from

Power-down and

Power-save

Additional Delay from

Reset

CC = 5.0V)

Recommended Usage

Crystal CL (pF)

Max ESR [k

Ω](1)

6.5

9.0

12.5

⋅

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