Data Sheet

32
8161D–AVR–10/09
ATmega48PA/88PA/168PA/328P
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.
ATmega48PA/88PA/168PA/328P oscillator is optimized for very low power consumption, and
thus when selecting crystals, see Table 8-7 on page 32 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 Oscillator
Note: 1. Maximum ESR is typical value based on characterization
The Low-frequency Crystal Oscillator provides an internal load capacitance at each TOSC pin
as specified in the Table 8-8 on page 32.
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 the ones given in the Table 8-8 on page
32, require external capacitors applied as described in Figure 8-2 on page 29.
The Low-frequency Crystal Oscillator must be selected by setting the CKSEL Fuses to “0110” or
“0111”, as shown in Table 8-10. Start-up times are determined by the SUT Fuses as shown in
Table 8-9.
Crystal CL (pF) Max ESR [kΩ]
(1)
6.5 75
9.0 65
12.5 30
Table 8-8. Capacitance for Low-Frequency Crystal Oscillator
Device 32 kHz Osc. Type Cap (Xtal1/Tosc1) Cap (Xtal2/Tosc2)
ATmega48PA/88PA/168PA/328P System Osc. 18 pF 8 pF
Timer Osc. 18 pF 8 pF
Table 8-9. 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
C 2 CL⋅ C
s
–=