When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 6-10.
Table 6-10. Start-up Times for the 128 kHz Internal Oscillator
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset
Power Conditions
BOD enabled
SUT1..0
00
6 CK
6 CK
14CK
Fast rising power
Slowly rising power
14CK + 4 ms
14CK + 64 ms
01
6 CK
10
Reserved
11
6.7
External Clock
The device can utilize a external clock source as shown in Figure 6-3. To run the device on an
external clock, the CKSEL Fuses must be programmed as shown in Table 6-1.
Figure 6-3. External Clock Drive Configuration
NC
XTAL2
EXTERNAL
CLOCK
XTAL1
GND
SIGNAL
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 6-11.
Table 6-11. Start-up Times for the External Clock Selection
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (VCC = 5.0V)
Power Conditions
BOD enabled
SUT1..0
00
6 CK
6 CK
14CK
Fast rising power
Slowly rising power
14CK + 4.1 ms
14CK + 65 ms
01
6 CK
10
Reserved
11
When applying an external clock, it is required to avoid sudden changes in the applied clock fre-
quency to ensure stable operation of the MCU. A variation in frequency of more than 2% from
one clock cycle to the next can lead to unpredictable behavior. If changes of more than 2% is
required, ensure that the MCU is kept in Reset during the changes.
Note that the System Clock Prescaler can be used to implement run-time changes of the internal
clock frequency while still ensuring stable operation. Refer to “System Clock Prescaler” on page
47 for details.
46
AT90USB64/128
7593A–AVR–02/06
ATMEL [ ATMEL ]
