AT90USB646_0607 datasheet(44/435 Pages) ATMEL

44 / 435 page

7593D–AVR–07/06

AT90USB64/128

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.

Note:

1. These options should only be used if frequency stability at start-up is not important for the

application.

6.5

Calibrated Internal RC Oscillator

The calibrated internal RC Oscillator by default provides a 8.0 MHz clock. The frequency is nom-

inal value at 3V and 25

°C. The device is shipped with the CKDIV8 Fuse programmed. See

“System Clock Prescaler” on page 48 for more details. This clock may be selected as the system

clock by programming the CKSEL Fuses as shown in Table 6-7. If selected, it will operate with

no external components. During reset, hardware loads the calibration byte into the OSCCAL

°C, this calibration

gives a frequency of 8 MHz ± 1%. The oscillator can be calibrated to any frequency in the range

7.3 - 8.1 MHz within ±1% accuracy, by changing the OSCCAL register. When this Oscillator is

used as the chip clock, the Watchdog Oscillator will still be used for the Watchdog Timer and for

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 (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(1)

Reserved

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

(VCC = 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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