AT90USB646 datasheet(46/434 Pages) ATMEL

46 / 434 page

7593A–AVR–02/06

AT90USB64/128

When this clock source is selected, start-up times are determined by the SUT Fuses as shown in

Table 6-10.

6.7

External Clock

The device can utilize a external clock source as shown in Figure 6-3. To run the device on an

external clock, the CKSEL Fuses must be programmed as shown in Table 6-1.

Figure 6-3.

External Clock Drive Configuration

When this clock source is selected, start-up times are determined by the SUT Fuses as shown in

Table 6-11.

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

47 for details.

Table 6-10.

Start-up Times for the 128 kHz Internal Oscillator

Power Conditions

Start-up Time from Power-

down and Power-save

Additional Delay from

Reset

SUT1..0

BOD enabled

6 CK

14CK

Fast rising power

6 CK

14CK + 4 ms

Slowly rising power

6 CK

14CK + 64 ms

Reserved

Table 6-11.

Start-up Times for the External 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

Reserved

EXTERNAL

CLOCK

SIGNAL

XTAL2

XTAL1

GND

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