ATMEGA169P-15AT Datasheet, PDF (32/373 Pages) ATMEL Corporation – 8-bit Microcontroller with 16K Bytes In-System Programmable Flash

7.3 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.
7.4 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 27-1
on page 327 and ”Internal Oscillator Speed” on page 349 for more deatils. 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 7-3. 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 cal-
ibrates the RC Oscillator. The accuracy of this calibration is shown as Factory calibration in
Table 27-1 on page 327.
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 27-1 on page 327.
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 296.
Table 7-3.
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 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 7-4
Table 7-4. Start-up times for the internal calibrated RC Oscillator clock selection
Power Conditions
BOD enabled
Start-up Time from Power-
down and Power-save
6 CK
Additional Delay from
Reset (V_CC = 5.0V)
14CK
Fast rising power
Slowly rising power
6 CK
6 CK
14CK + 4.1 ms
14CK + 65 ms⁽¹⁾
Reserved
Note: 1. The device is shipped with this option selected.
SUT1:0
00
01
10
11
32 ATmega169P Automotive
7735B–AVR–12/07