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 VCC 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.
Table 8-7.
Internal Calibrated RC Oscillator Operating Modes
CKSEL3:0
0001(1)
0010
Nominal Frequency (MHz)
1.0
2.0
4.0
8.0
0011
0100
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).
Table 8-8.
Start-up Times for the Internal Calibrated RC Oscillator Clock Selection
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (VCC = 5.0V)
SUT1:0
00
Recommended Usage
BOD enabled
6 CK
6 CK
6 CK
–
4.1 ms
01
Fast rising power
Slowly rising power
10(1)
65 ms
11
Reserved
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.
42
8160C–AVR–07/09
ATMEL [ ATMEL ]
