Datasheet

31
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
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.
Note: 1.
The device is shipped with this option selected.
6.4 Low Frequency Crystal Oscillator
The device can utilize a 32.768kHz watch crystal as clock source by a dedicated Low Frequency Crystal
Oscillator. The crystal should be connected as shown in Figure 6-2 on page 30. When this Oscillator is selected,
start-up times are determined by the SUT Fuses and CKSEL0 as shown in the table below.
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
Table 6-5. Start-up times for the internal calibrated RC Oscillator clock selection
Power Conditions
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (V
CC
= 5.0V) SUT1..0
BOD enabled 6CK 14CK 00
Fast rising power 6CK 14CK + 4.1ms 01
Slowly rising power 6CK 14CK + 65ms
(1)
10
Reserved 11
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
Table 6-6. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
Power Conditions
Start-up Time from
Power-down and
Power-save
Additional Delay from
Reset
(V
CC
= 5.0V) CKSEL0 SUT1..0
BOD enabled 1K CK 14CK
(1)
0 00
Fast rising power 1K CK 14CK + 4.1ms
(1)
0 01
Slowly rising power 1K CK 14CK + 65ms
(1)
0 10
Reserved 0 11
BOD enabled 32K CK 14CK 1 00
Fast rising power 32K CK 14CK + 4.1ms 1 01
Slowly rising power 32K CK 14CK + 65ms 1 10