33
8271D–AVR–05/11
ATmega48A/PA/88A/PA/168A/PA/328/P
9.5
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
t a k e n i n t o c o n s i d e r a t i o n . B o t h v a l u e s a r e s p e c i f i e d b y t h e c r y s t a l v e n d o r .
ATmega48A/PA/88A/PA/168A/PA/328/P oscillator is optimized for very low power consumption,
and thus when selecting crystals, see
for maximum ESR recommendations on 6.5pF,
9.0pF and 12.5pF crystals
Note:
1. Maximum ESR is typical value based on characterization
The Low-frequency Crystal Oscillator provides an internal load capacitance, see
at
each TOSC pin.
The capacitance (Ce+Ci) needed at each TOSC pin can be calculated by using:
where:
– Ce - is optional external capacitors as described in
– Ci - is the pin capacitance in
– CL - is the load capacitance for a 32.768kHz crystal specified by the crystal vendor
– CS - 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
.
The Low-frequency Crystal Oscillator must be selected by setting the CKSEL Fuses to “0110” or
“0111”, as shown in
. Start-up times are determined by the SUT Fuses as
shown in
.
Table 9-7.
Maximum ESR Recommendation for 32.768kHz Crystal
Crystal CL (pF)
Max ESR [k
Ω
]
6.5
75
9.0
65
12.5
30
Table 9-8.
Capacitance for Low-frequency Oscillator
Device
32kHz Osc. Type
Cap(Xtal1/Tosc1)
Cap(Xtal2/Tosc2)
ATmega48A/PA/88A/PA/168A/PA/3
28/P
System Osc.
18pF
8pF
Timer Osc.
18pF
8pF
Table 9-9.
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 + 4.1ms
Slowly rising power
10
4 CK + 65ms
Stable frequency at start-up
11
Reserved
C
2
CL
⋅
C
s
–
=