ATmega16U4 datasheet(34/408 Pages) ATMEL

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7766D–AVR–01/09

ATmega16U4/ATmega32U4

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. 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

37 for details.

Clock Switch

The ATmega16U4/ATmega32U4 product includes a Clock Switch controller, that allows user to

switch from one clock source to another one by software, in order to control application power

and execution time with more accuracy.

Example of use

The modification may be needed when the device enters in USB Suspend mode. It then

switches from External Clock to Calibrated RC Oscillator in order to reduce consumption and

wake-up delay. In such a configuration, the External Clock is disabled. The firmware can then

use the watchdog timer to be woken-up from power-down in order to check if there is an event

on the application. If an event occurs on the application or if the USB controller signals a non-

idle state on the USB line (Resume for example), the firmware switches the Clock Multiplexer

from the Calibrated RC Oscillator to the External Clock. in order to restart USB operation.

This feature can only be used to switch between Calibrated 8 MHz RC Oscillator, External Clock

and Low Power Crystal Oscillator. The Low Frequency Crystal Oscillator must not be used with

this feature.

Figure 1. Example of clock switching with wake-up from USB Host

Table 6-9.

Start-up Times for the External Clock Selection

Power Conditions

Start-up Time from Power-

down and Power-save

Additional Delay from

Reset (V

CC = 5.0V)

SUT1..0

BOD enabled

6 CK

14CK

Fast rising power

6 CK

14CK + 4.1 ms

Slowly rising power

6 CK

14CK + 65 ms

Reserved

USB

CPU Clock

External

Oscillator

RC oscillator

Ext

non-Idle

Idle

(Suspend)

non-Idle

resume

1 Resume from Host

w atchdog wake-up

fr om power- down

ATmega16U4 Datasheet(PDF) 34 Page - ATMEL Corporation

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