Datasheet

44
7593L–AVR–09/12
AT90USB64/128
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 fre-
quency 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.
7.4 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 7-2 on page 43. When this
Oscillator is selected, start-up times are determined by the SUT Fuses and CKSEL0 as shown in
Table 7-6.
Crystal Oscillator, BOD
enabled
16KCK 14CK 1 01
Crystal Oscillator, fast
rising power
16KCK 14CK + 4.1ms 1 10
Crystal Oscillator, slowly
rising power
16KCK 14CK + 65ms 1 11
Table 7-5. 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 7-4. Start-up times for the low power crystal oscillator clock selection. (Continued)
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 7-6. 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 1KCK 14CK
(1)
000
Fast rising power 1KCK 14CK + 4.1ms
(1)
001
Slowly rising power 1KCK 14CK + 65ms
(1)
010
Reserved 0 11
BOD enabled 32KCK 14CK 1 00
Fast rising power 32KCK 14CK + 4.1ms 1 01
Slowly rising power 32KCK 14CK + 65ms 1 10
Reserved 1 11