32
7734Q–AVR–02/12
AT90PWM81/161
The Oscillator can operate in three different modes, each optimized for a specific frequency
range. The operating mode is selected by CKSEL3..1 fuses or by CSEL3..1 field as shown in
Notes:
1. Flash fuse bits.
2. CLKSELR register bits.
3. This option should not be used with crystals, only with ceramic resonators.
The CKSEL0 Fuse together with the SUT1..0 Fuses or CSEL0 together with CSUT1..0 field
select the start-up times as shown in
Table 5-7.
Notes:
1. Flash fuse bits.
2. CLKSELR register bits.
3. 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.
4. 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 5-6.
Crystal oscillator operating modes.
Frequency range [MHz]
Recommended range for capacitors
C1 and C2 for use with crystals [pF]
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 5-7.
Start-up times for the crystal oscillator clock selection.
CKSEL0
Start-up time from
power-down and
power-save
Additional delay
from reset
(VCC = 5.0V)
Recommended usage
0
00
14CK + 4.1ms
Ceramic resonator,
fast rising power
0
01
14CK + 65ms
Ceramic resonator,
slowly rising power
0
10
14CK
Ceramic resonator,
BOD enabled
0
11
14CK + 4.1ms
Ceramic resonator,
fast rising power
1
00
14CK + 65ms
Ceramic resonator,
slowly rising power
1
01
16K (16384)CK
14CK
Crystal oscillator,
BOD enabled
1
10
16K (16384)CK
14CK + 4.1ms
Crystal oscillator,
fast rising power
1
11
16K (16384)CK
14CK + 65ms
Crystal oscillator,
slowly rising power