ATmega169P
7.3
7.4
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.
Calibrated Internal RC Oscillator
The calibrated internal RC Oscillator by default provides a 8.0 MHz clock. The frequency is nom-
inal value at 3V and 25°C. The device is shipped with the CKDIV8 Fuse programmed. See
”System Clock Prescaler” on page 36. for more details. This clock may be selected as the sys-
tem 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 calibration byte into the OSC-
CAL Register, see ”OSCCAL – Oscillator Calibration Register” on page 37, and thereby
automatically calibrates the RC Oscillator. At 3V and 25°C, this calibration gives a frequency of 8
MHz 1ꢀ. The oscillator can be calibrated to any frequency in the range 7.3 - 8.1 MHz within
1ꢀ accuracy, by changing the OSCCAL register. 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..
Table 7-3.
Internal Calibrated RC Oscillator Operating Modes(1)(3)
Frequency Range(2) (MHz)
CKSEL3:0
7.3 - 8.1
0010
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 VCC), 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
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (VCC = 5.0V)
Power Conditions
BOD enabled
SUT1:0
00
6 CK
6 CK
14CK
Fast rising power
Slowly rising power
14CK + 4.1 ms
14CK + 65 ms(1)
01
6 CK
10
Reserved
11
Note:
1. The device is shipped with this option selected.
31
8018A–AVR–03/06
ATMEL [ ATMEL ]
