Datasheet

33
8023F–AVR–07/09
ATmega325P/3250P
The Low-frequency Crystal Oscillator must be selected by setting the CKSEL Fuses to “0110” or
“0111” as shown in Table 8-7. Start-up times are determined by the SUT Fuses as shown in
Table 8-6.
Note: 1. This option should only be used if frequency stability at start-up is not important for the
application
8.6 Calibrated Internal RC Oscillator
By default, the Internal RC Oscillator provides an approximate 8 MHz clock. Though voltage and
temperature dependent, this clock can be very accurately calibrated by the user. See Table 26-1
on page 307 for more details. The device is shipped with the CKDIV8 Fuse programmed. See
”System Clock Prescaler” on page 35 for more details.
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in
Table 8-8 on page 33. 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 307.
By changing the OSCCAL register from SW, see ”OSCCAL – Oscillator Calibration Register” on
page 36, 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 26-1 on page 307.
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 cali-
bration value, see the section ”Calibration Byte” on page 274.
Notes: 1. The device is shipped with this option selected.
2. The frequency ranges are preliminary values. Actual values are TBD.
3. 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.
Table 8-6. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
SUT1..0 Additional Delay from Reset (V
CC
= 5.0V) Recommended Usage
00 4 CK Fast rising power or BOD enabled
01 4 CK + 4.1 ms Slowly rising power
10 4 CK + 65 ms Stable frequency at start-up
11 Reserved
Table 8-7. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
CKSEL3..0
Start-up Time from
Power-down and Power-save Recommended Usage
0110
(1)
1K CK
0111 32K CK Stable frequency at start-up
Table 8-8. Internal Calibrated RC Oscillator Operating Modes
(1)(3)
Frequency Range
(2)
(MHz) CKSEL3..0
7.3 - 8.1 0010