Datasheet
9.1.2. I/O Clock – clk
I/O
The I/O clock is used by the majority of the I/O modules, like Timer/Counters, SPI, and USART. The I/O
clock is also used by the External Interrupt module, but the start condition detection in the USI module is
carried out asynchronously when clk
I/O
is halted, TWI address recognition in all sleep modes.
Note: If a level triggered interrupt is used for wake-up from Power-down, the required level must be held
long enough for the MCU to complete the wake-up to trigger the level interrupt. If the level disappears
before the end of the Start-up Time, the MCU will still wake up, but no interrupt will be generated. The
start-up time is defined by the SUT and CKSEL Fuses.
9.1.3. Flash Clock – clk
FLASH
The Flash clock controls operation of the Flash interface. The Flash clock is usually active simultaneously
with the CPU clock.
9.1.4. PLL Clock – clk
PLL
The PLL clock allows the fast peripherals to be clocked directly from a 64/32MHz clock. A 16MHz clock is
also derived for the CPU.
9.1.5. Asynchronous Timer Clock – clk
ASY
The Asynchronous Timer clock allows Asynchronous Timer/Counters to be clocked directly from an
external clock. The dedicated clock domain allows using this Timer/Counter as a real-time counter even
when the device is in sleep mode.
9.1.6. ADC Clock – clk
ADC
The ADC is provided with a dedicated clock domain. This allows halting the CPU and I/O clocks in order
to reduce noise generated by digital circuitry. This gives more accurate ADC conversion results.
9.2. Clock Sources
The device has the following clock source options, selectable by Flash Fuse bits as shown below. The
clock from the selected source is input to the AVR clock generator, and routed to the appropriate
modules.
Table 9-1. Device Clocking Options Select
Device Clocking
Option
System clock PLL input CKSEL[3:0]
External crystal/ceramic
resonator
Ext osc RC osc 1111 - 1000
PLL output divided by 4 :
16MHz / PLL driven by
external crystal/ceramic
resonator
Ext osc Ext osc 0100
PLL output divided by 4 :
16MHz / PLL driven by
external crystal/ceramic
resonator
PLL/4 Ext osc 0101
Reserved N/A N/A 0110
Reserved N/A N/A 0111
Atmel ATmega16M1/32M1/64M1 [DATASHEET]
Atmel-8209F-ATmega16M1/32M1/64M1_Datasheet_Complete-10/2016
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