
ATmega48PA/88PA/168PA [DATASHEET]
9223F–AVR–04/14
28
Figure 9-3. Crystal Oscillator Connections
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 taken into consideration. Both values are specified by
the crystal vendor. Atmel ATmega48PA/88PA/168PA oscillator is optimized for very low power consumption, and thus
when selecting crystals, see
Table 9-7 for maximum ESR recommendations on 6.5pF, 9.0pF and 12.5pF crystals
Table 9-6.
Start-up Times for the Full Swing Crystal Oscillator Clock Selection
Oscillator Source / Power
Conditions
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset
(VCC = 5.0V)
CKSEL0
SUT1...0
Ceramic resonator, fast rising
power
258CK
14CK + 4.1ms(1)
0
00
Ceramic resonator, slowly rising
power
258CK
14CK + 65ms(1)
0
01
Ceramic resonator, BOD
enabled
1KCK
14CK(2)
0
10
Ceramic resonator, fast rising
power
1KCK
14CK + 4.1ms(2)
0
11
Ceramic resonator, slowly rising
power
1KCK
14CK + 65ms(2)
1
00
Crystal Oscillator, BOD enabled
16KCK
14CK
1
01
Crystal Oscillator, fast rising
power
16KCK
14CK + 4.1ms
1
10
Crystal Oscillator, slowly rising
power
16KCK
14CK + 65ms
1
11
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 frequency of the device, and if
frequency stability at start-up is not important for the application.
C2
XTAL2 (TOSC2)
XTAL1 (TOSC1)
GND
C1
Table 9-7.
Maximum ESR Recommendation for 32.768kHz Crystal
Crystal CL (pF)
Max ESR [k
Ω](1)
6.5
75
9.0
65
12.5
30
Notes: 1.
Maximum ESR is typical value based on characterization