
28
Atmel ATmega16/32/64/M1/C1 [DATASHEET]
7647K–AVR–12/13
The CKSEL0 Fuse together with the SUT1..0 Fuses select the start-up times as shown in
Table 5-4.Notes: 1.
These options should only be used when not operating close to the maximum frequency of the device, and only if
frequency stability at start-up is not important for the application. These options are not suitable for crystals.
2.
These options are intended for use with ceramic resonators and will ensure frequency stability at start-up. They
can also be used with crystals when not operating close to the maximum frequency of the device, and if frequency
stability at start-up is not important for the application.
5.5
Calibrated Internal RC Oscillator
By default, the Internal RC OScillator provides an approximate 8.0MHz clock. Though voltage and temperature dependent, this
clock can be very accurately calibrated by the user. The device is shipped with the CKDIV8 Fuse programmed. See
SectionThis clock may be selected as the system clock by programming the CKSEL Fuses as shown in
Table 5-1 on page 26. If
selected, it will operate with no external components. during reset, hardware loads the pre-programmed calibration value into
the OSCCAL Register and thereby automatically calibrates the RC oscillator. The accuracy of this calibration is shown as
possible to get a higher calibration accuracy than by using the factory calibration. The accuracy of this calibration is shown as
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 section.
Notes: 1.
The device is shipped with this option selected.
2.
If 8MHz frequency exceeds the specification of the device (depends on VCC), the CKDIV8 fuse can be pro-
grammed in order to divide the internal frequency by 8.
Table 5-4.
Start-up Times for the Oscillator Clock Selection
CKSEL0
SUT1..0
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (V
CC = 5.0V)
Recommended Usage
0
00
14CK + 4.1 ms
Ceramic resonator, fast rising
power
0
01
14CK + 65 ms
Ceramic resonator, slowly rising
power
0
10
14CK
Ceramic resonator, BOD enabled
0
11
14CK + 4.1 ms
Ceramic resonator, fast rising
power
1
00
14CK + 65 ms
Ceramic resonator, slowly rising
power
1
01
16K CK
14CK
Crystal Oscillator, BOD enabled
1
10
16K CK
14CK + 4.1 ms
Crystal Oscillator, fast rising
power
1
11
16K CK
14CK + 65 ms
Crystal Oscillator, slowly rising
power
Table 5-5.
Internal Calibrated RC Oscillator Operating Modes(1)(2) Frequency Range (MHz)
CKSEL3..0
7.3 - 8.1
0010