Datasheet

Table 10-8. Capacitance for Low-Frequency Oscillator
32kHz Osc. Type Cap. (XTAL1/TOSC1) Cap. (XTAL2/TSOC2)
System Osc. 18pF 8pF
Timer Osc. 6pF 6pF
The capacitance (C
e
+C
i
) needed at each TOSC pin can be calculated by using:


+ 

= 2

 

where:
• C
e
- is optional external capacitors.
• C
i
- is the pin capacitance in the above table.
• C
L
- is the load capacitance for a 32.768kHz crystal specified by the crystal vendor.
• C
S
- is the total stray capacitance for one TOSC pin.
Crystals specifying a load capacitance (CL) higher than 6pF require external capacitors applied as
described in Low Power Crystal Oscillator.
When this oscillator is selected, start-up times are determined by the SUT Fuses and CKSEL0 as shown
in the following table.
Table 10-9. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
Power Conditions Start-up Time from
Power-down and Power-
save
Additional Delay from Reset
(V
CC
= 5.0V)
CKSEL[0] SUT[1:0]
BOD enabled 1K CK 14CK
(1)
0 00
Fast rising power 1K CK 14CK + 4ms
(1)
0 01
Slowly rising power 1K CK 14CK + 65ms
(1)
0 10
Reserved 0 11
BOD enabled 32K CK 14CK 1 00
Fast rising power 32K CK 14CK + 4ms 1 01
Slowly rising power 32K CK 14CK + 65ms 1 10
Reserved 1 11
Note:
1. This option should only be used if frequency stability at start-up is not important for the application.
Related Links
Clock Source Connections on page 44
Timer/Counter Oscillator on page 51
10.6. Calibrated Internal RC Oscillator
By default, the Internal RC Oscillator provides an 8.0MHz clock. Though voltage and temperature
dependent, this clock can be very accurately calibrated by the user. The device is shipped with the
CKDIV8 Fuse programmed.
Atmel ATmega644A [DATASHEET]
Atmel-42716C-ATmega644A_Datasheet_Complete-10/2016
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