ATmega328P datasheet(32/448 Pages) ATMEL

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8161D–AVR–10/09

ATmega48PA/88PA/168PA/328P

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.

ATmega48PA/88PA/168PA/328P oscillator is optimized for very low power consumption, and

thus when selecting crystals, see Table 8-7 on page 32 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 Oscillator

Note:

1. Maximum ESR is typical value based on characterization

The Low-frequency Crystal Oscillator provides an internal load capacitance at each TOSC pin

as specified in the Table 8-8 on page 32.

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 the ones given in the Table 8-8 on page

32, require external capacitors applied as described in Figure 8-2 on page 29.

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

“0111”, as shown in Table 8-10. Start-up times are determined by the SUT Fuses as shown in

Table 8-9.

Crystal CL (pF)

Max ESR [k

Ω](1)

6.5

9.0

12.5

Table 8-8.

Capacitance for Low-Frequency Crystal Oscillator

Device

32 kHz Osc. Type

Cap (Xtal1/Tosc1)

Cap (Xtal2/Tosc2)

ATmega48PA/88PA/168PA/328P

System Osc.

18 pF

8 pF

Timer Osc.

18 pF

8 pF

Table 8-9.

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

2 CL

⋅

–

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