AT90USB646 datasheet(45/456 Pages) ATMEL

45 / 456 page

7593L–AVR–09/12

AT90USB64/128

Note:

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

application.

7.5

Calibrated internal RC oscillator

The calibrated internal RC oscillator by default provides a 8.0MHz clock. The frequency is nomi-

nal 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 7-7. 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 8MHz ±10%. The oscillator can be calibrated to any frequency in the range

7.3 - 8.1MHz 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 Section

“Calibration byte” on page 362

Notes:

1. The device is shipped with this option selected.

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

3. If 8MHz frequency exceeds the specification of the device (depends on V

CC), 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 7-5 on page 44.

Note:

1. The device is shipped with this option selected.

7.5.1

OSCCAL – Oscillator Calibration Register

• 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 automatically

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

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

Table 7-7.

Internal calibrated RC oscillator operating modes

(1)(3).

Frequency range (2) [MHz]

CKSEL3..0

7.3 - 8.1

0010

Table 7-8.

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 (VCC = 5.0V)

SUT1..0

BOD enabled

6CK

14CK

Fast rising power

6CK

14CK + 4.1ms

Slowly rising power

6CK

14CK + 65ms (1)

Reserved

Bit

7654

CAL7

CAL6

CAL5

CAL4

CAL3

CAL2

CAL1

CAL0

OSCCAL

Read/write

R/W

Initial value

Device specific calibration value

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