31
2570N–AVR–05/11
ATmega325/3250/645/6450
Figure 9-3.
External Clock Drive Configuration
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
When applying an external clock, it is required to avoid sudden changes in the applied clock fre-
quency 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
9.7
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 out-
put also during reset and the normal operation of I/O pin will be overridden 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.
9.8
Timer/Counter Oscillator
Atmel ATmega325/3250/645/6450 share the Timer/Counter Oscillator Pins (TOSC1 and
TOSC2) with XTAL1 and XTAL2. This means that the Timer/Counter Oscillator can only be used
when the calibrated internal RC Oscillator is selected as system clock source. The Oscillator is
connection.
Table 9-10.
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 + 4.1ms
Fast rising power
10
6 CK
14CK + 65ms
Slowly rising power
11
Reserved
NC
EXTERNAL
CLOCK
SIGNAL
XTAL2
XTAL1
GND