ATmega64A datasheet(42/392 Pages) ATMEL

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8160C–AVR–07/09

ATmega64A

8.7

Calibrated Internal RC Oscillator

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

cies 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 8-7. If selected, it will operate with no exter-

nal components. The CKOPT Fuse should always be unprogrammed 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, 25

⋅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 possi-

ble 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 preprogrammed calibration value, see the section

“Calibration Byte” on page 298.

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 8-8. XTAL1 and XTAL2 should be left unconnected (NC).

Note:

1. The device is shipped with this option selected.

8.8

External Clock

To drive the device from an external clock source, XTAL1 should be driven as shown in Figure

8-5. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.

By programming the CKOPT Fuse, the user can enable an internal 36 pF capacitor between

XTAL1 and GND.

Table 8-7.

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 8-8.

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

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