AT90USB646 datasheet(46/459 Pages) ATMEL

46 / 459 page

7593H–AVR–11/08

ATmega32U6/AT90USB64/128

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 6-9.

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.

6.7

Clock Output Buffer

The device can output the system clock on the CLKO pin. To enable the output, the CKOUT

Fuse has to be programmed. This mode is suitable when the chip clock is used to drive other cir-

cuits on the system. The clock also will be output during reset, and the normal operation of I/O

pin will be overridden when the fuse is programmed. Any clock source, including the internal RC

Oscillator, can be selected when the clock is output on CLKO. If the System Clock Prescaler is

used, it is the divided system clock that is output.

6.8

Timer/Counter Oscillator

The device can operate its Timer/Counter2 from an external 32.768 kHz watch crystal or a exter-

nal clock source. See Figure 6-2 on page 42 for crystal connection.

Applying an external clock source to TOSC1 requires EXCLK in the ASSR Register written to

logic one. See “Asynchronous operation of the Timer/Counter” on page 165 for further descrip-

tion on selecting external clock as input instead of a 32 kHz crystal.

Table 6-9.

Start-up Times for the External 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

Reserved

EXTERNAL

CLOCK

SIGNAL

XTAL2

XTAL1

GND

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