AT90USB646 datasheet(46/456 Pages) ATMEL

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7593L–AVR–09/12

AT90USB64/128

be calibrated to any frequency in the range 7.3 - 8.1MHz within ±10% accuracy. Calibration out-

side that range is not guaranteed.

Note that this oscillator is used to time EEPROM and Flash write accesses, and these write

times will be affected accordingly. If the EEPROM or Flash are written, do not calibrate to more

than 8.8MHz. Otherwise, the EEPROM or Flash write may fail.

The CAL7 bit determines the range of operation for the oscillator. Setting this bit to 0 gives the

lowest frequency range, setting this bit to 1 gives the highest frequency range. The two fre-

quency ranges are overlapping, in other words a setting of OSCCAL = 0x7F gives a higher

frequency than OSCCAL = 0x80.

The CAL6..0 bits are used to tune the frequency within the selected range. A setting of 0x00

gives the lowest frequency in that range, and a setting of 0x7F gives the highest frequency in the

range. Incrementing CAL6..0 by 1 will give a frequency increment of less than 2% in the fre-

quency range 7.3 - 8.1MHz.

7.6

External clock

The device can utilize a external clock source as shown in Figure 7-3. To run the device on an

external clock, the CKSEL fuses must be programmed as shown in Table 7-1 on page 41.

Figure 7-3.

External clock drive configuration.

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

Table 7-9.

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.

Table 7-9.

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

6CK

14CK

Fast rising power

6CK

14CK + 4.1ms

Slowly rising power

6CK

14CK + 65ms

Reserved

EXTERNAL

CLOCK

SIGNAL

XTAL2

XTAL1

GND

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