29
8154B–AVR–07/09
ATmega16A
Note:
1. This option should not be used when operating close to the maximum frequency of the device.
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 fre-
quencies 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 8-8. 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, 25
°C and 1.0, 2.0, 4.0 or 8.0 MHz
Oscillator frequency selected, this calibration gives a frequency within ± 3% of the nominal fre-
quency. 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 cali-
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 8-9. XTAL1 and XTAL2 should be left unconnected (NC).
Note:
1. The device is shipped with this option selected.
Table 8-7.
Start-up Times for the External 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
00
18 CK
–
BOD enabled
01
18 CK
4.1 ms
Fast rising power
10
18 CK
65 ms
Slowly rising power
11
4.1 ms
Fast rising power or BOD enabled
Table 8-8.
Internal Calibrated RC Oscillator Operating Modes
CKSEL3:0
Nominal Frequency (MHz)
1.0
0010
2.0
0011
4.0
0100
8.0
Table 8-9.
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
00
6 CK
–
BOD enabled
01
6 CK
4.1 ms
Fast rising power
6 CK
65 ms
Slowly rising power
11
Reserved