Datasheet

32
8018P–AVR–08/10
ATmega169P
8.3 Default Clock Source
The device is shipped with CKSEL = “0010”, SUT = “10”, and CKDIV8 programmed. The default
clock source setting is the Internal RC Oscillator with longest start-up time and an initial system
clock prescaling of 8. This default setting ensures that all users can make their desired clock
source setting using an In-System or Parallel programmer.
8.4 Calibrated Internal RC Oscillator
B 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 28-2
on page 332 and ”Internal Oscillator Speed” on page 365 for more details. The device is shipped
with the CKDIV8 Fuse programmed. See ”System Clock Prescaler” on page 37 for more details.
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in
Table 8-3. 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 cali-
brates the RC Oscillator. The accuracy of this calibration is shown as Factory calibration in Table
28-2 on page 332.
By changing the OSCCAL register from SW, see ”OSCCAL – Oscillator Calibration Register” on
page 38, 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 28-2 on page 332.
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 299.
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.
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Table 8-4.
Note: 1. The device is shipped with this option selected.
Table 8-3. Internal Calibrated RC Oscillator Operating Modes
(1)(3)
Frequency Range
(2)
(MHz) CKSEL3:0
7.3 - 8.1 0010
Table 8-4. Start-up times for the internal calibrated RC Oscillator clock selection
Power Conditions
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (V
CC
= 5.0V) SUT1:0
BOD enabled 6 CK 14CK 00
Fast rising power 6 CK 14CK + 4.1 ms 01
Slowly rising power 6 CK 14CK + 65 ms
(1)
10
Reserved 11