ATmega640/1280/1281/2560/2561
128 kHz Internal
Oscillator
The 128 kHz internal Oscillator is a low power Oscillator providing a clock of 128 kHz.
The frequency is nominal at 3V and 25°C. This clock may be select as the system clock
by programming the CKSEL Fuses to “11” as shown in Table 16.
Table 16. 128 kHz Internal Oscillator Operating Modes
Nominal Frequency
CKSEL3..0
128 kHz
0011
Note:
1. The frequency is preliminary value. Actual value is TBD.
When this clock source is selected, start-up times are determined by the SUT Fuses as
shown in Table 17.
Table 17. Start-up Times for the 128 kHz Internal Oscillator
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset
Power Conditions
BOD enabled
SUT1..0
00
6 CK
6 CK
14CK
Fast rising power
Slowly rising power
14CK + 4 ms
14CK + 64 ms
01
6 CK
10
Reserved
11
External Clock
To drive the device from an external clock source, XTAL1 should be driven as shown in
Figure 24. To run the device on an external clock, the CKSEL Fuses must be pro-
grammed to “0000”.
Figure 24. External Clock Drive Configuration
NC
XTAL2
EXTERNAL
CLOCK
XTAL1
GND
SIGNAL
When this clock source is selected, start-up times are determined by the SUT Fuses as
shown in Table 20..
Table 18. Crystal Oscillator Clock Frequency
Nominal Frequency
CKSEL3..0
0 - 16 MHz
0000
47
2549A–AVR–03/05
ATMEL [ ATMEL ]
