Datasheet
2. These options are intended for use with ceramic resonators and will ensure frequency stability at
start-up. They can also be used with crystals when not operating close to the maximum frequency
of the device, and if frequency stability at start-up is not important for the application.
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13.2.3 Clock Source Connections
13.4 Full Swing Crystal Oscillator
13.4 Full Swing Crystal Oscillator
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 for the Low
Power Crystal Oscillator. Note that the full swing crystal oscillator will only operate for V
CC
=2.7-5.5V.
Some initial guidelines for choosing capacitors for use with crystals are given in Table 13-6. The crystal
should be connected as described in Clock Source Connections”.
The operating mode is selected based on the fuses CKSEL[3:1] as shown in the table:
Table 13-5. Full Swing Crystal Oscillator operating modes
Frequency Range
(1)
[MHz] CKSEL[3:1] Absolute limits for Capacitors C1 and C2 [pF]
0.4 - 20 011 12 - 22
Note:
1. If the crystal 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.
For the crystal oscillator connections refer to Low Power Crystal Oscillator in the previous section.
Table 13-6. 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 SUT[1:0]
Ceramic resonator, fast rising
power
258 CK 14CK + 4.1 ms
(1)
0 00
Ceramic resonator, slowly rising
power
258 CK 14CK + 65 ms
(1)
0 01
Ceramic resonator, BOD enabled 1K CK 14CK
(2)
0 10
Ceramic resonator, fast rising
power
1K CK 14CK + 4.1 ms
(2)
0 11
Ceramic resonator, slowly rising
power
1K CK 14CK + 65 ms
(2)
1 00
Crystal Oscillator, BOD enabled 16K CK 14CK 1 01
ATmega48PA/88PA/168PA
System Clock and Clock Options
© 2018 Microchip Technology Inc.
Datasheet Complete
DS40002011A-page 58