Datasheet

34
8284D–AVR–6/11
ATmega169A/PA/329A/PA/3290A/PA/649A/P/6490A/P
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.
9.6 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.
ATmega329A/329PA oscillator is optimized for very low power consumption, and thus when
selecting crystals, see Table 9-7 for maximum ESR recommendations on 9pF and 6.5pF
crystals
Table 9-7. Maximum ESR Recommendation for 32.768kHz Watch Crystal
ATmega169A/169PA
Note: 1. Maximum ESR is typical value based on characterization
The Low-frequency Crystal Oscillator provides an internal load capacitance, see Table 9-9 on
page 35 at each TOSC pin.
ATmega169A/169PA/329A/329PA/649A/649P/3290A/3290PA/6490A/6490P oscillator is opti-
mized for very low power consumption, and thus when selecting crystals, see Table 9-8 on page
34 for maximum ESR recommendations on 6.5pF, 9.0pF and 12.5pF crystals.
Table 9-8. Maximum ESR Recommendation for 32.768kHz Watch Crystal
ATmega329A/329PA/3290A/3290PA/649A/649P/6490A/6490P
Note: 1. Maximum ESR is typical value based on characterization
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 9-6. 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 60
935
Crystal CL (pF) Max ESR [kΩ]
(1)
6.5 75
9.0 65
12.5 30