Datasheet

ATmega48A/PA/88A/PA/168A/PA/328/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002061A-page 40
This Crystal Oscillator is a full swing oscillator, with rail-to-rail swing on the XTAL2 output. This is useful for
driving other clock inputs and in noisy environments. The current consumption is higher than the ”Low Power
Crystal Oscillator” on page 38. Note that the Full Swing Crystal Oscillator will only operate for V
CC
= 2.7 - 5.5
volts.
C1 and C2 should always be equal for both crystals and resonators. The optimal value of the capacitors
depends on the crystal 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 crystals are given in Table 9-6 on
page 40. For ceramic resonators, the capacitor values given by the manufacturer should be used.
The operating mode is selected by the fuses CKSEL3...1 as shown in Table 9-5.
Notes: 1. If the cryatal 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. It must be ensured that the resulting divided clock
meets the frequency specification of the device.
Figure 9-3. Crystal Oscillator Connections
Table 9-5. Full Swing Crystal Oscillator operating modes
Frequency Range
(1)
(MHz)
Recommended Range for
Capacitors C1 and C2 (pF)
CKSEL3...1
0.4 - 20 12 - 22 011
Table 9-6. Start-up Times for the Full Swing Crystal Oscillator Clock Selection
Oscillator Source /
Power Conditions
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(V
CC
= 5.0V) CKSEL0 SUT1...0
Ceramic resonator, fast
rising power
258 CK 14CK + 4.1ms
(1)
0 00
Ceramic resonator, slowly
rising power
258 CK 14CK + 65ms
(1)
0 01
Ceramic resonator, BOD
enabled
1K CK 14CK
(2)
0 10
Ceramic resonator, fast
rising power
1K CK 14CK + 4.1ms
(2)
0 11
Ceramic resonator, slowly
rising power
1K CK 14CK + 65ms
(2)
1 00
XTAL2 (TOSC2)
XTAL1 (TOSC1)
GND
C2
C1