ATMEGA2561V ATMEL [ATMEL Corporation], ATMEGA2561V Datasheet - Page 46

• Manufacturer Part Number: ATMEGA2561V
• Description: 8-bit Microcontroller with 64K/128K/256K Bytes In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

Clock Output Buffer
Timer/Counter Oscillator
46
ATmega640/1280/1281/2560/2561
When this clock source is selected, start-up times are determined by the SUT Fuses as
shown in Table 23.
Table 21. Crystal Oscillator Clock Frequency
Table 22. 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. If changes
of more than 2% is required, ensure that the MCU is kept in Reset during the changes.
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 47 for details.
The device can output the system clock on the CLKO pin. To enable the output, the
CKOUT Fuse has to be programmed. This mode is suitable when the chip clock is used
to drive other circuits on the system. The clock also will be output during reset, and the
normal operation of I/O pin will be overridden when the fuse is programmed. Any clock
source, including the internal RC Oscillator, can be selected when the clock is output on
CLKO. If the System Clock Prescaler is used, it is the divided system clock that is
output.
The device can operate its Timer/Counter2 from an external 32.768 kHz watch crystal or
a external clock source. See Figure 21 on page 40 for crystal connection.
Applying an external clock source to TOSC1 requires EXCLK in the ASSR Register writ-
ten to logic one. See “Asynchronous Operation of Timer/Counter2” on page 188 for
further description on selecting external clock as input instead of a 32 kHz crystal.
BOD enabled
Fast rising power
Slowly rising power
Power Conditions
Nominal Frequency
0 - 16 MHz
Start-up Time from Power-
down and Power-save
Reserved
6 CK
6 CK
6 CK
Additional Delay from
Reset (V
14CK + 4.1 ms
14CK + 65 ms
14CK
CC
CKSEL3:0
= 5.0V)
0000
2549K–AVR–01/07
SUT1:0
00
01
10
11