Datasheet

Oscillator Source / Power
Conditions
Start-Up Time from
Power-down and
Power-save
Additional Delay from Reset
(V
CC
= 5.0V)
CKSEL0 SUT[1:0]
Crystal oscillator, fast rising power 16K CK 14CK + 4.1 ms 1 10
Crystal oscillator, slowly rising
power
16K CK 14CK + 65 ms 1 11
Note:
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 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.
Related Links
13.2.3 Clock Source Connections
13.3 Low-Power Crystal Oscillator
13.5 Low-Frequency Crystal Oscillator
The low-frequency crystal oscillator is optimized for use with a 32.768 kHz watch crystal. When selecting
crystals, load capacitance and crystal’s Equivalent Series Resistance (ESR) must be taken into
consideration. Both values are specified by the crystal vendor. The oscillator is optimized for very low
power consumption, and thus when selecting crystals, consider the maximum ESR recommendations:
Table 13-7. Maximum ESR Recommendation for 32.768kHz Crystal
Crystal CL [pF] Max. ESR [kΩ]
(1)
6.5 75
9.0 65
12.5 30
Note:
1. Maximum ESR is typical value based on characterization.
The low-frequency crystal oscillator provides an internal load capacitance at each TOSC pin:
Table 13-8. Capacitance for Low-Frequency Oscillator
32kHz Osc. Type Cap. (XTAL1/TOSC1) Cap. (XTAL2/TOSC2)
C
i
of system oscillator (XTAL pins) 18 pF 8 pF
C
i
of timer oscillator (TOSC pins) 18 pF 8 pF
The capacitance (Ce+Ci) needed at each TOSC pin can be calculated by using:
 = 2C  

where:
ATmega48PA/88PA/168PA
System Clock and Clock Options
© 2018 Microchip Technology Inc.
Datasheet Complete
DS40002011A-page 59