Datasheet

45
ATmega640/1280/1281/2560/2561
2549K–AVR–01/07
Note: 1. The device is shipped with this option selected.
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 19.
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 20.
External Clock To drive the device from an external clock source, XTAL1 should be driven as shown in
Figure 23. To run the device on an external clock, the CKSEL Fuses must be pro-
grammed to “0000”.
Figure 23. External Clock Drive Configuration
Table 18. Start-up times for the internal calibrated RC Oscillator clock selection
Power Conditions
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (V
CC
= 5.0V) SUT1:0
BOD enabled 6 CK 14CK 00
Fast rising power 6 CK 14CK + 4.1 ms 01
Slowly rising power 6 CK 14CK + 65 ms
(1)
10
Reserved 11
Table 19. 128 kHz Internal Oscillator Operating Modes
Nominal Frequency CKSEL3:0
128 kHz 0011
Table 20. Start-up Times for the 128 kHz Internal Oscillator
Power Conditions
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset SUT1:0
BOD enabled 6 CK 14CK 00
Fast rising power 6 CK 14CK + 4 ms 01
Slowly rising power 6 CK 14CK + 64 ms 10
Reserved 11
NC
EXTERNAL
CLOCK
SIGNAL
XTAL2
XTAL1
GND