ATMEGA8U2_14 Datasheet, PDF (31/310 Pages) ATMEL Corporation – 125 Powerful Instructions – Most Single Clock Cycle Execution

ATmega8U2/16U2/32U2
3. If 8 MHz 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.
The CKSEL0 Fuse together with the SUT1..0 Fuses select the start-up times as shown in Table
8-4.
Table 8-4. Start-up Times for the Low Power Crystal Oscillator Clock Selection
Oscillator Source /
Power Conditions
Ceramic resonator, fast
rising power
Start-up Time from
Power-down and
Power-save
258 CK
Additional Delay
from Reset
(V_CC = 5.0V)
14CK + 4.1 ms⁽¹⁾
CKSEL0
0
Ceramic resonator, slowly
rising power
258 CK
14CK + 65 ms⁽¹⁾
0
Ceramic resonator, BOD
enabled
1K CK
14CK⁽²⁾
0
Ceramic resonator, fast
rising power
1K CK
14CK + 4.1 ms⁽²⁾
0
Ceramic resonator, slowly
rising power
1K CK
14CK + 65 ms⁽²⁾
1
Crystal Oscillator, BOD
enabled
16K CK
14CK
1
Crystal Oscillator, fast
rising power
16K CK
14CK + 4.1 ms
1
Crystal Oscillator, slowly
rising power
16K CK
14CK + 65 ms
1
SUT1..0
00
01
10
11
00
01
10
11
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.
Table 8-5. Start-up times for the internal calibrated RC Oscillator clock selection
Power Conditions
BOD enabled
Start-up Time from Power-
down and Power-save
6 CK
Additional Delay from
Reset (V_CC = 5.0V)
14CK
Fast rising power
Slowly rising power
6 CK
6 CK
14CK + 4.1 ms
14CK + 65 ms⁽¹⁾
Reserved
Note: 1. The device is shipped with this option selected.
SUT1..0
00
01
10
11
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