Low-Frequency Crystal Oscillator - Atmel ATtiny25 Manual

6.5

Low-frequency Crystal Oscillator

7598H–AVR–07/09
Table 6-4. Start-up Times for the Crystal Oscillator Clock Selection
CKSEL0 SUT1..0
0 00
0 01
0 10
0 11
1 00
1 01
1 10
1 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.
To use a 32.768 kHz watch crystal as the clock source for the device, the low-frequency crystal
oscillator must be selected by setting CKSEL fuses to '0110'. The crystal should be connected
as shown in Figure 6-3. Refer to the 32 kHz Crystal Oscillator Application Note for details on
oscillator operation and how to choose appropriate values for C1 and C2.
When this oscillator is selected, start-up times are determined by the SUT fuses as shown in
Table 6-5.
Table 6-5. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
Start-up Time from
Power Down and Power
SUT1..0 Save
00 1K CK
01 1K CK
10 32K CK
11
Notes: 1. These options should only be used if frequency stability at start-up is not important for the
application.
Start-up Time from
Power-down and
Power-save
(1)
258 CK
(1)
258 CK
(2)
1K CK
(2)
1K CK
(2)
1K CK
16K CK
16K CK
16K CK
Additional Delay from
Power On Reset
(V
CC
(1)
4 ms
(1)
64 ms
64 ms
Reserved
ATtiny25/45/85
Additional Delay
from Reset
(V = 5.0V) Recommended Usage
CC
Ceramic resonator, fast
14CK + 4 ms
rising power
Ceramic resonator, slowly
14CK + 64 ms
rising power
Ceramic resonator, BOD
14CK
enabled
Ceramic resonator, fast
14CK + 4 ms
rising power
Ceramic resonator, slowly
14CK + 64 ms
rising power
Crystal Oscillator, BOD
14CK
enabled
Crystal Oscillator, fast
14CK + 4 ms
rising power
Crystal Oscillator, slowly
14CK + 64 ms
rising power
= 5.0V) Recommended usage
Fast rising power or BOD
enabled
Slowly rising power
Stable frequency at start-up
25