Datasheet

30
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
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 the below
table. For ceramic resonators, the capacitor values given by the manufacturer should be used.
Figure 6-2. Crystal Oscillator Connections
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 CKSEL[3..1] as shown in this table.
Notes: 1. This option should not be used with crystals, only with ceramic resonators.
2. If 8 MHz frequency exceeds the specification of the device (depends on VCC), 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.
The CKSEL0 Fuse together with the SUT1..0 Fuses select the start-up times as shown in this table.
Table 6-3. Low Power Crystal Oscillator Operating Modes
Frequency Range
(1)
[MHz] CKSEL3..1 Recommended Range for Capacitors C1 and C2 [pF]
0.4 - 0.9 100
(2)
–
0.9 - 3.0 101 12 - 22
3.0 - 8.0 110 12 - 22
8.0 - 16.0 111 12 - 22
Table 6-4. 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
XTAL2
XTAL1
GND
C2
C1