AT90USB646 datasheet(44/456 Pages) ATMEL

44 / 456 page

7593L–AVR–09/12

AT90USB64/128

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.

7.4

Low frequency crystal oscillator

The device can utilize a 32.768kHz watch crystal as clock source by a dedicated low frequency

crystal oscillator. The crystal should be connected as shown in Figure 7-2 on page 43. When this

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

Table 7-6.

Crystal Oscillator, BOD

enabled

16KCK

14CK

Crystal Oscillator, fast

rising power

16KCK

14CK + 4.1ms

Crystal Oscillator, slowly

rising power

16KCK

14CK + 65ms

Table 7-5.

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 (V

CC = 5.0V)

SUT1..0

BOD enabled

6CK

14CK

Fast rising power

6CK

14CK + 4.1ms

Slowly rising power

6CK

14CK + 65ms (1)

Reserved

Table 7-4.

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 7-6.

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

1KCK

14CK (1)

000

Fast rising power

1KCK

14CK + 4.1ms (1)

001

Slowly rising power

1KCK

14CK + 65ms (1)

010

Reserved

BOD enabled

32KCK

14CK

Fast rising power

32KCK

14CK + 4.1ms

Slowly rising power

32KCK

14CK + 65ms

Reserved

AT90USB646 Datasheet(PDF) 44 Page - ATMEL Corporation

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