Datasheet
33
4317K–AVR–03/2013
AT90PWM2/3/2B/3B
The Oscillator can operate in three different modes, each optimized for a specific frequency
range. The operating mode is selected by the fuses CKSEL3..1 as shown in Table 7-4.
Notes: 1. The frequency ranges are preliminary values. Actual values are TBD.
2. This option should not be used with crystals, only with ceramic resonators.
The CKSEL0 Fuse together with the SUT1..0 Fuses select the start-up times as shown in Table
7-5.
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.
7.5 Calibrated Internal RC Oscillator
By default, the Internal RC OScillator provides an approximate 8.0 MHz clock. Though voltage
and temperature dependent, this clock can be very accurately calibrated by the user. The device
is shipped with the CKDIV8 Fuse programmed. See “System Clock Prescaler” on page 38 for
more details.
Table 7-4. Crystal Oscillator Operating Modes
CKSEL3..1 Frequency Range
(1)
(MHz)
Recommended Range for Capacitors C1 and
C2 for Use with Crystals (pF)
100
(2)
0.4 - 0.9 –
101 0.9 - 3.0 12 - 22
110 3.0 - 8.0 12 - 22
111 8.0 -16.0 12 - 22
Table 7-5. Start-up Times for the Oscillator Clock Selection
CKSEL0 SUT1..0
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(V
CC
= 5.0V) Recommended Usage
0 00 258 CK
(1)
14CK + 4.1 ms
Ceramic resonator, fast
rising power
0 01 258 CK
(1)
14CK + 65 ms
Ceramic resonator, slowly
rising power
010 1K CK
(2)
14CK
Ceramic resonator, BOD
enabled
011 1K CK
(2)
14CK + 4.1 ms
Ceramic resonator, fast
rising power
100 1K CK
(2)
14CK + 65 ms
Ceramic resonator, slowly
rising power
1 01 16K CK 14CK
Crystal Oscillator, BOD
enabled
1 10 16K CK 14CK + 4.1 ms
Crystal Oscillator, fast
rising power
1 11 16K CK 14CK + 65 ms
Crystal Oscillator, slowly
rising power