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, see
Table 8-6 onTable 8-6.
Capacitance for Low-frequency Oscillator
011
14CK + 4.1ms
Ceramic resonator, fast
rising power
100
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 [k
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