ATmega3290P datasheet(31/385 Pages) ATMEL

31 / 385 page

8021E–AVR–07/09

ATmega329P/3290P

The Low-frequency Crystal Oscillator must be selected by setting the CKSEL Fuses to “0110” or

“0111” as shown in Table 8-7. Start-up times are determined by the SUT Fuses as shown in

Table 8-6.

Note:

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

application

8.6

Calibrated Internal RC Oscillator

By default, the Internal RC Oscillator provides an approximate 8 MHz clock. Though voltage and

temperature dependent, this clock can be very accurately calibrated by the user. The device is

shipped with the CKDIV8 Fuse programmed. See ”System Clock Prescaler” on page 33 for

more details.

This clock may be selected as the system clock by programming the CKSEL Fuses as shown in

Table 8-8 on page 31. If selected, it will operate with no external components. During reset,

hardware loads the pre-programmed calibration value into the OSCCAL Register and thereby

automatically calibrates the RC Oscillator. The accuracy of this calibration is shown as Factory

calibration in Table 27-1 on page 329.

By changing the OSCCAL register from SW, see ”OSCCAL – Oscillator Calibration Register” on

page 34, it is possible to get a higher calibration accuracy than by using the factory calibration.

The accuracy of this calibration is shown as User calibration in Table 27-1 on page 329.

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 cali-

bration value, see the section ”Calibration Byte” on page 296.

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.

Table 8-6.

Start-up Times for the Low-frequency Crystal Oscillator Clock Selection

SUT1..0

Additional Delay from Reset (VCC = 5.0V)

Recommended Usage

4 CK

Fast rising power or BOD enabled

4 CK + 4.1 ms

Slowly rising power

4 CK + 65 ms

Stable frequency at start-up

Reserved

Table 8-7.

Start-up Times for the Low-frequency Crystal Oscillator Clock Selection

CKSEL3..0

Start-up Time from

Power-down and Power-save

Recommended Usage

0110(1)

1K CK

0111

32K CK

Stable frequency at start-up

Table 8-8.

Internal Calibrated RC Oscillator Operating Modes

(1)(3)

Frequency Range(2) (MHz)

CKSEL3..0

7.3 - 8.1

0010

ATmega3290P Datasheet(PDF) 31 Page - ATMEL Corporation

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