ATMEGA64-16AI datasheet(43/393 Pages) ATMEL

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ATmega64(L)

2490L–AVR–10/06

Calibrated Internal RC

Oscillator

The calibrated internal RC Oscillator provides a fixed 1.0, 2.0, 4.0, or 8.0 MHz clock. All

frequencies are nominal values at 5V and 25

°C. This clock may be selected as the sys-

tem 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 unpro-

grammed when using this clock option. During reset, hardware loads the calibration byte

into the OSCCAL Register and thereby automatically calibrates the RC Oscillator. At 5V,

°C and 1.0 MHz Oscillator frequency selected, this calibration gives a frequency

within ± 3% of the nominal frequency. Using run-time calibration methods as described

in application notes available at www.atmel.com/avr it is possible to achieve ± 1% accu-

racy 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 preprogrammed calibration value, see the section

“Calibration Byte” on page 294.

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).

Note:

1. The device is shipped with this option selected.

OSCCAL – Oscillator

Calibration Register

(1)

Note:

1. The 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 pro-

cess variations 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 calibration values in the Flash or EEPROM. Then the

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 (V

CC = 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

7654

321

(0x6F)

CAL7

CAL6

CAL5

CAL4

CAL3

CAL2

CAL1

CAL0

OSCCAL

Read/Write

R/W

Initial Value

Device Specific Calibration Value

ATMEGA64-16AI Datasheet(PDF) 43 Page - ATMEL Corporation

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