ATTINY13 datasheet(23/177 Pages) ATMEL

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ATtiny13

2535G–AVR–01/07

Calibrated Internal RC

Oscillator

The calibrated internal RC Oscillator provides an 9.6 MHz or 4.8 MHz clock. The fre-

quency 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 25. for more details. This clock may be selected as the system clock by program-

ming the CKSEL Fuses as shown in Table 4. If selected, it will operate with no external

components. During reset, hardware loads the calibration byte into the OSCCAL Regis-

ter and thereby automatically calibrates the RC Oscillator. At 3V and 25

°C, this

calibration gives a frequency within ± 10% of the nominal frequency. Using calibration

methods as described in application notes available at www.atmel.com/avr it is possible

to achieve ± 3% accuracy at any given V

CC and Temperature. 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 106.

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 5..

Note:

1. The device is shipped with this option selected.

Oscillator Calibration Register

– OSCCAL

• Bit 7 – Res: Reserved Bit

This bit is reserved bit in the ATtiny13 and will always read as zero.

• Bits 6..0 – CAL6..0: Oscillator Calibration Value

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

cess variations 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 register will increase the frequency of the internal Oscillator. Writing

0x7F 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 cali-

brate to more than 10% above the nominal frequency. Otherwise, the EEPROM or Flash

Table 4. Internal Calibrated RC Oscillator Operating Modes

CKSEL1..0

Nominal Frequency

(1)

9.6 MHz

4.8 MHz

Table 5. Start-up Times for the Internal Calibrated RC Oscillator Clock Selection

SUT1..0

Start-up Time

from Power-down

Additional Delay from

Reset (V

CC = 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

Bit

7654

321

–

CAL6

CAL5

CAL4

CAL3

CAL2

CAL1

CAL0

OSCCAL

Read/Write

R/W

Initial Value

Device Specific Calibration Value

ATTINY13 Datasheet(PDF) 23 Page - ATMEL Corporation

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