AT90USB647 datasheet(43/456 Pages) ATMEL

43 / 456 page

7593L–AVR–09/12

AT90USB64/128

Figure 7-2.

Crystal oscillator connections.

The low power oscillator can operate in three different modes, each optimized for a specific fre-

quency range. The operating mode is selected by the fuses CKSEL3..1 as shown in Table 7-3.

Notes:

1. The frequency ranges are preliminary values. Actual values are TBD.

2. This option should not be used with crystals, only with ceramic resonators.

3. If 8MHz frequency exceeds the specification of the device (depends on V

CC), the CKDIV8

Fuse can be programmed in order to divide the internal frequency by 8. It must be ensured

that the resulting divided clock meets the frequency specification of the device.

The CKSEL0 Fuse together with the SUT1..0 fuses select the start-up times as shown in Table

7-4.

Table 7-3.

Low power crystal oscillator operating modes (3).

Frequency range (1) [MHz]

CKSEL3..1

Recommended range for capacitors C1

and C2 [pF]

0.4 - 0.9

100 (2)

–

0.9 - 3.0

101

12 - 22

3.0 - 8.0

110

12 - 22

8.0 - 16.0

111

12 - 22

Table 7-4.

Start-up times for the low power crystal oscillator clock selection.

Oscillator source /

power conditions

Start-up time from

power-down and

power-save

Additional delay

from reset

CC = 5.0V)

CKSEL0

SUT1..0

Ceramic resonator, fast

rising power

258CK

14CK + 4.1ms (1)

000

Ceramic resonator, slowly

rising power

258CK

14CK + 65ms (1)

001

Ceramic resonator, BOD

enabled

1KCK

14CK (2)

010

Ceramic resonator, fast

rising power

1KCK

14CK + 4.1ms (2)

011

Ceramic resonator, slowly

rising power

1KCK

14CK + 65ms (2)

100

XTAL2

XTAL1

GND

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