AT90USB646 datasheet(45/459 Pages) ATMEL

45 / 459 page

7593H–AVR–11/08

ATmega32U6/AT90USB64/128

Note:

1. The device is shipped with this option selected.

6.5.1

Oscillator Calibration Register – OSCCAL

• Bits 7..0 – CAL7..0: Oscillator Calibration Value

The Oscillator Calibration Register is used to trim the Calibrated Internal RC Oscillator to

remove process variations from the oscillator frequency. The factory-calibrated value is automat-

ically written to this register during chip reset, giving an oscillator frequency of 8.0 MHz at 25°C.

The application software can write this register to change the oscillator frequency. The oscillator

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

outside 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 8.8 MHz. 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 - 8.1 MHz.

6.6

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.

Table 6-8.

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

Bit

CAL7

CAL6

CAL5

CAL4

CAL3

CAL2

CAL1

CAL0

OSCCAL

Read/Write

R/W

Initial Value

Device Specific Calibration Value

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