Datasheet

31
7810C–AVR–10/12
Atmel ATmega328P [Preliminary]
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. ATmega328P
oscillator is optimized for very low power consumption, and thus when selecting crystals, see
Table 8-7 on page 31 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
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 28.
The Low-frequency Crystal Oscillator must be selected by setting the CKSEL Fuses to “0110” or
“0111”, as shown in Table 8-9. Start-up times are determined by the SUT Fuses as shown in
Table 8-8.
Note: 1. This option should only be used if frequency stability at start-up is not important for the
application
Crystal CL (pF) Max ESR [kΩ]
(1)
6.5 75
9.0 65
12.5 30
Table 8-8. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
SUT1..0 Additional Delay from Reset (V
CC
= 5.0V) Recommended Usage
00 4 CK Fast rising power or BOD enabled
01 4 CK + 4.1 ms Slowly rising power
10 4 CK + 65 ms Stable frequency at start-up
11 Reserved
Table 8-9. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
CKSEL3..0
Start-up Time from
Power-down and Power-save Recommended Usage
0100
(1)
1K CK
0101 32K CK Stable frequency at start-up
C2CLC
S
–×=