29
ATmega16M1/32M1/64M1 [DATASHEET]
8209E–AVR–11/2012
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 sta-
bility at start-up is not important for the application.
8.5
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
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in
Table 8-1 on page27. If selected, it will operate with no external components. During reset, hardware loads the pre-programmed cali-
bration value into the OSCCAL Register and thereby automatically calibrates the RC Oscillator. The accuracy of
sible to get a higher calibration accuracy than by using the factory calibration. The accuracy of this calibration is
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 eight.
Table 8-4.
Start-up times for the oscillator clock selection
CKSEL0
SUT1..0
Start-up time from
power-down and
power-save
Additional delay
from reset
(VCC = 5.0V)
Recommended usage
0
00
14CK + 4.1ms
Ceramic resonator, fast
rising power
0
01
14CK + 65ms
Ceramic resonator, slowly
rising power
010
14CK
Ceramic resonator, BOD
enabled
011
14CK + 4.1ms
Ceramic resonator, fast
rising power
100
14CK + 65ms
Ceramic resonator, slowly
rising power
1
01
16K CK
14CK
Crystal oscillator, BOD
enabled
1
10
16K CK
14CK + 4.1ms
Crystal oscillator, fast
rising power
1
11
16K CK
14CK + 65ms
Crystal oscillator, slowly
rising power
Table 8-5.
Internal calibrated RC oscillator operating modes
(1)(2).Frequency range [MHz]
CKSEL3..0
7.3 - 8.1
0010