Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 41
9.3 Low Power Crystal Oscillator
This Crystal Oscillator is a low power oscillator, with reduced voltage swing on the XTAL2 output. It gives the
lowest power consumption, but is not capable of driving other clock inputs, and may be more susceptible to
noise in noisy environments. In these cases, refer to the ”Full swing Crystal Oscillator” on page 42.
Some initial guidelines for choosing capacitors for use with crystals are given in Table 9-3. The crystal should be
connected as described in ”Clock source connections” on page 40.
The Low Power Oscillator can operate in three different modes, each optimized for a specific frequency range.
The operating mode is selected by the fuses CKSEL3..1 as shown in Table 9-3.
Notes: 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.
2. This is the recommended CKSEL settings for the different frequency ranges.
3. This option should not be used with crystals, only with ceramic resonators.
The CKSEL0 Fuse together with the SUT1..0 Fuses select the start-up times as shown in Table 9-4.
Notes: 1. These options should only be used when not operating close to the maximum frequency of the device, and
only if frequency stability at start-up is not important for the application. These options are not suitable for
crystals.
Table 9-3. Low Power Crystal Oscillator Operating Modes
(1)
Frequency Range [MHz] CKSEL3..1
(2)
Recommended Range for Capacitors C1 and C2 [pF]
0.4 - 0.9 100
(3)
–
0.9 - 3.0 101 12 - 22
3.0 - 8.0 110 12 - 22
8.0 - 16.0 111 12 - 22
Table 9-4. Start-up Times for the Low Power 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
258CK 14CK + 4.1ms
(1)
0 00
Ceramic resonator, slowly
rising power
258CK 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
Crystal Oscillator, BOD
enabled
16K CK 14CK 1 01
Crystal Oscillator, fast
rising power
16K CK 14CK + 4.1ms 1 10
Crystal Oscillator, slowly
rising power
16K CK 14CK + 65ms 1 11