Datasheet

32
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
S
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
1
01 16K CK 14CK Crystal Oscillator, BOD
enabled
1
10 16K CK 14CK + 4.1 ms Crystal Oscillator, fast
rising power
1
11 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
(V
CC
= 5.0V) Recommended Usage
Crystal CL (pF) Max ESR [kΩ]
(1)
6.5 75
9.0 65
12.5 30
C 2 CL⋅ C
s
–=