Datasheet

Device Clocking
Option
System clock PLL input CKSEL[3:0]
PLL output divided by 4 :
16MHz
PLL/4 RC osc 0011
Calibrated Internal RC
Oscillator
RC osc RC osc 0010
PLL output divided by 4 :
16MHz / PLL driven by
external clock
PLL/4 Ext clk 0001
External Clock Ext clk RC osc 000
Note: For all fuses, '1' means unprogrammed while '0' means programmed.
The various choices for each clocking option is given in the following sections. When the CPU wakes up
from Power-down or Power-save, the selected clock source is used to time the start-up, ensuring stable
Oscillator operation before instruction execution starts. When the CPU starts from reset, there is an
additional delay allowing the power to reach a stable level before starting normal operation. The
Watchdog Oscillator is used for timing this realtime part of the start-up time. The number of WDT
Oscillator cycles used for each time-out is shown below.
Table 9-2. Number of watchdog oscillator cycles
Typical time-out (V
CC
= 5.0V) Typical time-out (V
CC
= 3.0V) Number of cycles
4.1ms 4.3ms 4K (4,096)
65ms 69ms 64K (65,536)
9.3. Default Clock Source
The device is shipped with CKSEL = “0010”, SUT = “10”, and CKDIV8 programmed. The default clock
source setting is the Internal RC Oscillator with longest start-up time and an initial system clock
prescaling of eight. This default setting ensures that all users can make their desired clock source setting
using an In-System or Parallel programmer.
9.4. Low Power Crystal Oscillator
XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be configured for
use as an On-chip Oscillator, as shown in the figure below. Either a quartz crystal or a ceramic resonator
may be used.
This Crystal Oscillator is a low power oscillator, with reduced voltage swing on the XTAL2 output. It gives
the lowest power consumption, but is not capable of driving other clock inputs.
C1 and C2 should always be equal for both crystals and resonators. The optimal value of the capacitors
depends on the crystal or resonator in use, the amount of stray capacitance, and the electromagnetic
noise of the environment. Some initial guidelines for choosing capacitors for use with crystals are given in
the table below. For ceramic resonators, the capacitor values given by the manufacturer should be used.
For more information on how to choose capacitors and other details on Oscillator operation, refer to the
Multi-purpose Oscillator Application Note.
Atmel ATmega16M1/32M1/64M1 [DATASHEET]
Atmel-8209F-ATmega16M1/32M1/64M1_Datasheet_Complete-10/2016
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