Datasheet

Note:
1. The 128kHz oscillator is a very low power clock source, and is not designed for high accuracy.
When this clock source is selected, start-up times are determined by the SUT Fuses:
Table 10-13. Start-Up Times for the 128kHz Internal Oscillator
Power Conditions Start-Up Time from Power-down and Power-
save
Additional Delay from Reset SUT[1:0]
BOD enabled 6 CK 14CK 00
Fast rising power 6 CK 14CK + 4ms 01
Slowly rising power 6 CK 14CK + 65ms 10
Reserved 11
10.8. External Clock
To drive the device from an external clock source, EXTCLK should be driven as shown in the Figure
below. To run the device on an external clock, the CKSEL Fuses must be programmed to '0000':
Table 10-14. External Clock Frequency
Frequency
(1)
CKSEL[3:0]
0 - 20MHz 0000
Note:
1. If the cryatal frequency exceeds the specification of the device (depends on VCC), the CKDIV8
Fuse can be programmed in order to divide the internal frequency by 8. It must be ensured that the
resulting divided clock meets the frequency specification of the device.
Figure 10-3. External Clock Drive Configuration
EXTERNAL
CLOCK
SIGNAL
EXTCLK
GND
When this clock source is selected, start-up times are determined by the SUT Fuses:
Table 10-15. Start-Up Times for the External Clock Selection - SUT
Power Conditions Start-Up Time from Power-down
and Power-save
Additional Delay from Reset (V
CC
=
5.0V)
SUT[1:0]
BOD enabled 6 CK 14CK 00
Fast rising power 6 CK 14CK + 4ms 01
Atmel ATmega644A [DATASHEET]
Atmel-42716C-ATmega644A_Datasheet_Complete-10/2016
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