AT90USB646 datasheet(44/459 Pages) ATMEL

44 / 459 page

7593H–AVR–11/08

ATmega32U6/AT90USB64/128

Note:

1. These options should only be used if frequency stability at start-up is not important for the

application.

6.5

Calibrated Internal RC Oscillator

The calibrated internal RC Oscillator by default provides a 8.0 MHz clock. The frequency is nom-

inal value at 3V and 25

⋅C. The device is shipped with the CKDIV8 Fuse programmed. See

“System Clock Prescaler” on page 47 for more details. This clock may be selected as the system

clock by programming the CKSEL Fuses as shown in Table 6-7. If selected, it will operate with

no external components. During reset, hardware loads the calibration byte into the OSCCAL

⋅C, this calibration

gives a frequency of 8 MHz ±10%. The oscillator can be calibrated to any frequency in the range

7.3 - 8.1 MHz within ±10% accuracy, by changing the OSCCAL register. When this Oscillator is

used as the chip clock, the Watchdog Oscillator will still be used for the Watchdog Timer and for

the Reset Time-out. For more information on the pre-programmed calibration value, see the sec-

tion “Calibration Byte” on page 369

Notes:

1. The device is shipped with this option selected.

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

3. If 8 MHz frequency exceeds the specification of the device (depends on VCC), the CKDIV8

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

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

Table 6-5 on page 43.

Table 6-7.

Internal Calibrated RC Oscillator Operating Modes

(1)(3)

Frequency Range

(2) (MHz)

CKSEL3..0

7.3 - 8.1

0010

AT90USB646 Datasheet(PDF) 44 Page - ATMEL Corporation