The CKSEL0 Fuse together with the SUT1..0 Fuses select the start-up times as shown in Table 5-4.
Table 5-4. Start-up Times for the Oscillator Clock Selection
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (VCC = 5.0V)
CKSEL0
SUT1..0
Recommended Usage
Ceramic resonator, fast rising
power
0
00
258 CK(1)
14CK + 4.1ms
Ceramic resonator, slowly rising
power
0
0
0
01
10
11
258 CK(1)
1K CK(2)
1K CK(2)
14CK + 65ms
14CK
Ceramic resonator, BOD enabled
Ceramic resonator, fast rising
power
14CK + 4.1ms
Ceramic resonator, slowly rising
power
1
1
1
00
01
10
1K CK(2)
16K CK
16K CK
14CK + 65ms
14CK
Crystal Oscillator, BOD enabled
Crystal Oscillator, fast rising
power
14CK + 4.1ms
Crystal Oscillator, slowly rising
power
1
11
16K CK
14CK + 65ms
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 fre-
quency 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
Section 5.10 “System Clock Prescaler” on page 32 for more details.
This 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 factory calibration in Table 26-1 on page 276.
By changing the OSCCAL register from SW, see Section 5.5.1 “Oscillator Calibration Register – OSCCAL” on page 29, it is
possible to get a higher calibration accuracy than by using the factory calibration. The accuracy of this calibration is shown
as User calibration in Section 26.3 “Clock Characteristics” on page 276.
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.
Table 5-5. Internal Calibrated RC Oscillator Operating Modes(1)(2)
Frequency Range (MHz)
7.3 - 8.1
CKSEL3..0
0010
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.
28
ATmega16/32/64/M1/C1 [DATASHEET]
7647O–AVR–01/15
MICROCHIP [ MICROCHIP ]
