ATtiny25 datasheet(26/196 Pages) ATMEL

26 / 196 page

7598H–AVR–07/09

ATtiny25/45/85

6.6

Calibrated Internal RC Oscillator

The calibrated internal RC Oscillator provides an 8.0 MHz clock. The frequency is the nominal

value at 3V and 25°C. If the frequency exceeds the specification of the device (depends on V

CC),

the CKDIV8 Fuse must be programmed in order to divide the internal frequency by 8 during

start-up. See “System Clock Prescaler” on page 29. for more details. This clock may be selected

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

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

OSCCAL Register and thereby automatically calibrates the RC Oscillator. At 3V and 25°C, this

calibration gives a frequency within ± 1% of the nominal frequency. 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 section

“Calibration Byte” on page 137.

Note:

1. The device is shipped with this option selected.

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

Table 6-7.

Note:

1. The device is shipped with this option selected.

6.6.1

Oscillator Calibration Register – OSCCAL

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

Writing the calibration byte to this address will trim the internal Oscillator to remove process vari-

ations from the Oscillator frequency. This is done automatically during Chip Reset. When

OSCCAL is zero, the lowest available frequency is chosen. Writing non-zero values to this regis-

ter will increase the frequency of the internal Oscillator. Writing 0xFF to the register gives the

highest available frequency. The calibrated Oscillator is used to time EEPROM and Flash

access. If EEPROM or Flash is written, do not calibrate to more than 8.8 MHz frequency. Other-

wise, the EEPROM or Flash write may fail.

Table 6-6.

Internal Calibrated RC Oscillator Operating Modes

CKSEL3..0

Nominal Frequency

0010

(1)

8.0 MHz

Table 6-7.

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 + 4 ms

BOD enabled

6 CK

14CK + 4 ms

Fast rising power

10(1)

6 CK

14CK + 64 ms

Slowly rising power

Reserved

Bit

765

432

CAL7

CAL6

CAL5

CAL4

CAL3

CAL2

CAL1

CAL0

OSCCAL

Read/Write

R/W

Initial Value

Device Specific Calibration Value

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