Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 44
When this oscillator is selected, start-up times are determined by the SUT Fuses and CKSEL0 as shown in
Table 9-9.
Note: 1. These options should only be used if frequency stability at start-up is not important for the application.
9.6 Calibrated Internal RC Oscillator
By default, the Internal RC Oscillator provides an approximate 8MHz clock. Though voltage and temperature
dependent, this clock can be very accurately calibrated by the user. See Table 28-10 on page 333 and ”Internal
oscillator speed” on page 389 and page 441 for more details. The device is shipped with the CKDIV8 Fuse
programmed. See ”System Clock Prescaler” on page 47 for more details.
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in Table 9-10. 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 28-10 on page 333.
By changing the OSCCAL register from SW, see ”OSCCAL – Oscillator Calibration Register” on page 48, 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 Table 28-10 on page 333.
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
”Calibration byte” on page 298.
Notes: 1. The device is shipped with this option selected.
2. If 8MHz 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.
Table 9-9. Start-up times for the Low Frequency Crystal Oscillator Clock Selection
Power conditions
Start-up time from
power-down and
power-save
Additional delay
from reset
(V
CC
= 5.0V) CKSEL0 SUT1..0
BOD enabled 1K CK 14CK
(1)
0 00
Fast rising power 1K CK 14CK + 4.1ms
(1)
0 01
Slowly rising power 1K CK 14CK + 65ms
(1)
0 10
Reserved 0 11
BOD enabled 32K CK 14CK 1 00
Fast rising power 32K CK 14CK + 4.1ms 1 01
Slowly rising power 32K CK 14CK + 65ms 1 10
Reserved 1 11
Table 9-10. Internal Calibrated RC Oscillator Operating Modes
Frequency range
(2)
[MHz] CKSEL3..0
7.3 - 8.1 0010
(1)