Datasheet

31
ATmega169A/PA/329A/PA/649A/P/3290A/PA/6490A/P [DATASHEET]
8284E–AVR–02/2013
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in Table 9-3. 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 calibrates the RC Oscillator. The accuracy of this cali-
bration is shown as Factory calibration in Table 29-12 on page 333.
By changing the OSCCAL register from SW, see ”OSCCAL – Oscillator Calibration Register” on page 37, it is pos-
sible to get a higher calibration accuracy than by using the factory calibration. The accuracy of this calibration is
shown as User calibration in Table 29-12 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 ”Cali-
bration Byte” on page 296.
Notes: 1. The device is shipped with this option selected.
2. The frequency ranges are preliminary values.
3. If 8MHz frequency exceeds the specification of the device (depends on V
CC
), the CKDIV8 Fuse can be pro-
grammed 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 9-4.
Note: 1. The device is shipped with this option selected.
9.5 XTAL oscillator
XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be configured for use as
an On-chip Oscillator, as shown in Figure 9-2. Either a quartz XTAL or a ceramic resonator may be used.
C1 and C2 should always be equal for both XTALs and resonators. The optimal value of the capacitors depends on
the XTAL or resonator in use, the amount of stray capacitance, and the electromagnetic noise of the environment.
Some initial guidelines for choosing capacitors for use with XTALs are given in Table 9-5. For ceramic resonators,
the capacitor values given by the manufacturer should be used.
Figure 9-2. XTAL oscillator connections.
Table 9-3. Internal calibrated RC oscillator operating modes
(1)(3)
.
Frequency range
(2)
[MHz] CKSEL3...0
7.3 - 8.1 0010
Table 9-4. 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 6CK 14CK 00
Fast rising power 6CK 14CK + 4.1ms 01
Slowly rising power 6CK 14CK + 65ms
(1)
10
Reserved 11
XTAL2
XTAL1
GND
C2
C1