ATmega128 datasheet(42/386 Pages) ATMEL

42 / 386 page

2467S–AVR–07/09

ATmega128

Calibrated Internal

RC Oscillator

The Calibrated Internal RC Oscillator provides a fixed 1.0, 2.0, 4.0, or 8.0 MHz clock. All fre-

quencies are nominal values at 5V and 25

°C. This clock may be selected as the system clock by

programming the CKSEL fuses as shown in Table 13. If selected, it will operate with no external

components. The CKOPT fuse should always be unprogrammed when using this clock option.

During Reset, hardware loads the calibration byte for the 1MHz oscillator into the OSCCAL Reg-

ister and thereby automatically calibrates the RC Oscillator. At 5V, 25

°C and 1.0 MHz Oscillator

frequency selected, this calibration gives a frequency within ± 3% of the nominal frequency.

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

possible to achieve ± 1% 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 sec-

tion “Calibration Byte” on page 289.

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 14. XTAL1 and XTAL2 should be left unconnected (NC).

Notes:

1. The device is shipped with this option selected.

Oscillator Calibration

Note:

OSCCAL Register is not available in ATmega103 compatibility mode.

• 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. During Reset, the 1 MHz calibration value which is located

in the signature row high byte (address 0x00) is automatically loaded into the OSCCAL Register.

If the internal RC is used at other frequencies, the calibration values must be loaded manually.

This can be done by first reading the signature row by a programmer, and then store the calibra-

tion values in the Flash or EEPROM. Then the value can be read by software and loaded into

the OSCCAL Register. 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 $FF

Table 13. Internal Calibrated RC Oscillator Operating Modes

CKSEL3..0

Nominal Frequency (MHz)

0001(1)

1.0

0010

2.0

0011

4.0

0100

8.0

Table 14. Start-up Times for the Internal Calibrated RC Oscillator Clock Selection

SUT1..0

Start-up Time from Power-

down and Power-save

Additional Delay from

Reset (VCC = 5.0V)

Recommended Usage

6 CK

–

BOD enabled

6 CK

4.1 ms

Fast rising power

10(1)

6 CK

65 ms

Slowly rising power

Reserved

Bit

765

4321

CAL7

CAL6

CAL5

CAL4

CAL3

CAL2

CAL1

CAL0

OSCCAL

Read/Write

R/W

Initial Value

Device Specific Calibration Value

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