Datasheet
30
8021G–AVR–03/11
ATmega329P/3290P
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 32.768kHz watch crystal. When
selecting crystals, load capacitance and crystal’s Equivalent Series Resistance, ESR must be
taken into consideration. Both values are specified by the crystal vendor.
ATmega329P/3290P oscillator is optimized for very low power consumption, and thus when
selecting crystals, see Table 8-5 for maximum ESR recommendations on 6.5pF, 9.0pF and
12.5pF crystals.
Table 8-5. Maximum ESR Recommendation for 32.768kHz Watch Crystal
ATmega329P/3290P
Note: 1. Maximum ESR is typical value based on characterization
The Low-frequency Crystal Oscillator provides an internal load capacitance, seeTable 8-6 on
page 30 at each TOSC pin.
Table 8-6. Capacitance for Low-frequency Oscillator
011 1K CK
(2)
14CK + 4.1ms Ceramic resonator, fast
rising power
100 1K CK
(2)
14CK + 65ms Ceramic resonator,
slowly rising power
1
01 16K CK 14CK Crystal Oscillator, BOD
enabled
1
10 16K CK 14CK + 4.1ms Crystal Oscillator, fast
rising power
1
11 16K CK 14CK + 65ms 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
(V
CC
= 5.0V) Recommended Usage
Crystal CL (pF) Max ESR [kΩ]
(1)
6.5 75
9.0 65
12.5 30
Device 32kHz Osc. Type Cap(Xtal1/Tosc1) Cap(Xtal2/Tosc2)
ATmega329P/3290P System Osc. 16pF 6pF
Timer Osc. 16pF 6pF