Datasheet
Table 11-9. Start-Up Times for the Low Frequency Crystal Oscillator Clock Selection - CKSEL
Fuses
CKSEL[3:0] Start-Up Time from
Power-Down and Power-Save
Recommended Usage
0100
(1)
1K CK
0101 32K CK Stable frequency at start-up
Note:
1. This option should only be used if frequency stability at start-up is not important for the application.
11.5 Calibrated Internal RC Oscillator
By default, the internal RC oscillator provides an 8.0 MHz 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.
This clock may be selected as the system clock by programming the CKSEL fuses as shown in the
following table. 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.
By changing the OSCCAL register from SW, it is possible to get a higher calibration accuracy than by
using the factory calibration.
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
section Calibration Byte.
Table 11-10. Internal Calibrated RC Oscillator Operating Modes
Frequency Range
(1)
[MHz] CKSEL[3:0]
7.6 - 8.4 0010
(2)
Note:
1. If 8 MHz frequency exceeds the specification of the device (depends on V
CC
), the CKDIV8 Fuse
can be programmed in order to divide the internal frequency by 8.
2. The device is shipped with this option selected.
Warning: The oscillator frequency is not guaranteed to be monotonic within the given range as the
oscillator calibration contains discontinuity (see figure 8 MHz RC Oscillator Frequency vs. OSCCAL Value
in chapter Typical Characteristics.)
When this oscillator is selected, start-up times are determined by the SUT fuses:
Table 11-11. Start-Up Times for the Internal Calibrated RC Oscillator Clock Selection - SUT
Power Conditions Start-Up Time from Power-Down
and Power-Save
Additional Delay from Reset (V
CC
= 5.0V) SUT[1:0]
BOD enabled 6 CK 19CK
(1)
00
Fast rising power 6 CK 19CK + 4 ms 01
ATmega328PB
System Clock and Clock Options
© 2018 Microchip Technology Inc.
Datasheet Complete
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