Datasheet

30
ATmega325/3250/645/6450
2570K–AVR–04/07
External Clock To drive the device from an external clock source, XTAL1 should be driven as shown in
Figure 14. To run the device on an external clock, the CKSEL Fuses must be pro-
grammed to “0000”.
Figure 14. External Clock Drive Configuration
When this clock source is selected, start-up times are determined by the SUT Fuses as
shown in Table 12.
When applying an external clock, it is required to avoid sudden changes in the applied
clock frequency to ensure stable operation of the MCU. A variation in frequency of more
than 2% from one clock cycle to the next can lead to unpredictable behavior. It is
required to ensure that the MCU is kept in Reset during such changes in the clock
frequency.
Note that the System Clock Prescaler can be used to implement run-time changes of
the internal clock frequency while still ensuring stable operation. Refer to “System Clock
Prescaler” on page 31 for details.
Clock Output Buffer When the CKOUT Fuse is programmed, the system Clock will be output on CLKO. This
mode is suitable when the chip clock is used to drive other circuits on the system. The
clock will be output also during reset and the normal operation of I/O pin will be overrid-
den when the fuse is programmed. Any clock source, including internal RC Oscillator,
can be selected when CLKO serves as clock output. If the System Clock Prescaler is
used, it is the divided system clock that is output when the CKOUT Fuse is
programmed.
Table 11. Crystal Oscillator Clock Frequency
CKSEL3..0 Frequency Range
0000 0 - 16 MHz
Table 12. Start-up Times for the External Clock Selection
SUT1..0
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (V
CC
= 5.0V) Recommended Usage
00 6 CK 14CK BOD enabled
01 6 CK 14CK + 4.1 ms Fast rising power
10 6 CK 14CK + 65 ms Slowly rising power
11 Reserved
NC
EXTERNAL
CLOCK
SIGNAL
XTAL2
XTAL1
GND