ATMEGA325-16AJ ATMEL [ATMEL Corporation], ATMEGA325-16AJ Datasheet - Page 30

• Manufacturer Part Number: ATMEGA325-16AJ
• Description: 8-bit Microcontroller with In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

Clock Output Buffer
Timer/Counter Oscillator
30
ATmega325/3250/645/6450
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.
Table 11. Crystal Oscillator Clock Frequency
Table 12. Start-up Times for the External Clock Selection
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.
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.
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.
SUT1..0
00
01
10
11
Start-up Time from Power-
CKSEL3..0
down and Power-save
0000
EXTERNAL
6 CK
6 CK
6 CK
SIGNAL
CLOCK
NC
Additional Delay from
Reset (V
Reserved
14CK + 4.1 ms
14CK + 65 ms
14CK
XTAL2
XTAL1
GND
CC
Frequency Range
= 5.0V)
0 - 16 MHz
Recommended Usage
BOD enabled
Fast rising power
Slowly rising power
2570A–AVR–09/04