Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 42
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.
9.4 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 the ”Low Power
Crystal Oscillator” on page 41. Note that the Full Swing Crystal Oscillator will only operate for Vcc = 2.7 - 5.5
volts.
Some initial guidelines for choosing capacitors for use with crystals are given in Table 9-6. The crystal should be
connected as described in ”Clock source connections” on page 40.
The operating mode is selected by the fuses CKSEL3..1 as shown in Table 9-5.
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.
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.
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.
Table 9-5. Full Swing Crystal Oscillator Operating Modes
Frequency range
(1)
[MHz] CKSEL3..1 Recommended Range for Capacitors C1 and C2 [pF]
0.4 - 20 011 12 - 22
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
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