ATtiny261-15MZ datasheet(29/227 Pages) ATMEL

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7753C–AVR–07/09

ATtiny261/ATtiny461/ATtiny861

7.6

Calibrated Internal RC Oscillator

By default, the Internal RC Oscillator provides an approximate 8.0 MHz clock. Though voltage

and temperature dependent, this clock can be very accurately calibrated by the user. See Table

23-1 on page 189 and “Internal Oscillator Speed” on page 207 for more details. The device is

shipped with the CKDIV8 Fuse programmed. See “System Clock Prescaler” on page 32 for

more details.

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

Table 7-6 on page 29. 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 23-1 on page 189.

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

page 32, 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 23-1 on page 189.

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 173.

Notes:

1. The device is shipped with this option selected.

2. 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 7-7 on page 29.

Note:

1. The device is shipped with this option selected.

Table 7-6.

Internal Calibrated RC Oscillator Operating Modes

(1)(2)

Frequency Range (MHz)

CKSEL3..0

7.84 - 8.16

0010

Table 7-7.

Start-up Times for the Internal Calibrated RC Oscillator Clock Selection

SUT1..0

Start-up Time

from Power-down

Additional Delay from

Reset (VCC = 5.0V)

Recommended Usage

6 CK

14CK

BOD enabled

6 CK

14CK + 4 ms

Fast rising power

10(1)

6 CK

14CK + 64 ms

Slowly rising power

Reserved

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