ATmega16U4 datasheet(31/408 Pages) ATMEL

31 / 408 page

7766D–AVR–01/09

ATmega16U4/ATmega32U4

Notes:

1. These options should only be used when not operating close to the maximum frequency of the

device, and only if frequency stability at start-up is not important for the application. These

options are not suitable for crystals.

2. These options are intended for use with ceramic resonators and will ensure frequency stability

at start-up. They can also be used with crystals when not operating close to the maximum fre-

quency of the device, and if frequency stability at start-up is not important for the application.

Note:

The device is shipped with this option selected.

6.4

Low Frequency Crystal Oscillator

The device can utilize a 32.768 kHz watch crystal as clock source by a dedicated Low Fre-

quency Crystal Oscillator. The crystal should be connected as shown in Figure 6-2. When this

Oscillator is selected, start-up times are determined by the SUT Fuses and CKSEL0 as shown in

Table 6-6.

Crystal Oscillator, BOD

enabled

16K CK

14CK

Crystal Oscillator, fast

rising power

16K CK

14CK + 4.1 ms

Crystal Oscillator,

slowly rising power

16K CK

14CK + 65 ms

Table 6-5.

Start-up times for the internal calibrated RC Oscillator clock selection

Power Conditions

Start-up Time from Power-

down and Power-save

Additional Delay from

Reset (VCC = 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(1)

Reserved

Table 6-4.

Start-up Times for the Low Power Crystal Oscillator Clock Selection (Continued)

Oscillator Source /

Power Conditions

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset

CC = 5.0V)

CKSEL0

SUT1..0

Table 6-6.

Start-up Times for the Low Frequency Crystal Oscillator Clock Selection

Power Conditions

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset

CC = 5.0V)

CKSEL0

SUT1..0

BOD enabled

1K CK

14CK(1)

000

Fast rising power

1K CK

14CK + 4.1 ms(1)

001

Slowly rising power

1K CK

14CK + 65 ms(1)

010

Reserved

BOD enabled

32K CK

14CK

Fast rising power

32K CK

14CK + 4.1 ms

Slowly rising power

32K CK

14CK + 65 ms

Reserved

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