Datasheet

33
8025M–AVR–6/11
ATmega48P/88P/168P
9.5 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.
ATmega48P/88P/168P oscillator is optimized for very low power consumption, and thus when
selecting crystals, see Table 9-7 on page 33 for maximum ESR recommendations on 6.5 pF,
9.0 pF and 12.5 pF crystals
Table 9-7. 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 9-2 on page 30.
The Low-frequency Crystal Oscillator must be selected by setting the CKSEL Fuses to “0110” or
“0111”, as shown in Table 9-10. Start-up times are determined by the SUT Fuses as shown in
Table 9-9.
Crystal CL (pF) Max ESR [kΩ]
(1)
6.5 75
9.0 65
12.5 30
Table 9-8. Capacitance for the Low-frequency Crystal Oscillator
Device
32 kHz Osc.
Type
Cap
(Xtal1/Tosc1)
Cap
(Xtal2/Tosc2)
ATmega48P/88P/169P
System Osc. 18 pF 8 pF
Timer Osc. 18 pF 8 pF
Table 9-9. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
SUT[1:0] 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
–=