Datasheet

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
(V
CC
= 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]
(1)
6.5 75
9.0 65
12.5 30
Notes: 1. Maximum ESR is typical value based on characterization