Datasheet

28
7701F–AVR–10/12
Atmel ATtiny24/44/84 [Preliminary]
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 stabil-
ity at start-up. They can also be used with crystals when not operating close to the maximum
frequency of the device, and if frequency stability at start-up is not important for the
application.
7.5 Low-frequency Crystal Oscillator
To use a 32.768kHz 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 7-2. See the 32kHz Crystal Oscillator Application Note for details on oscil-
lator 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 7-5.
Notes: 1. These options should only be used if frequency stability at start-up is not important for the
application.
Table 7-4. Start-up Times for the Crystal 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
000 258 CK
(1)
14CK + 4.1ms
Ceramic resonator, fast
rising power
001 258 CK
(1)
14CK + 65ms
Ceramic resonator,
slowly rising power
010 1K CK
(2)
14CK
Ceramic resonator, BOD
enabled
011 1K CK
(2)
14CK + 4.1ms
Ceramic resonator, fast
rising power
100 1K CK
(2)
14CK + 65ms
Ceramic resonator,
slowly rising power
1 01 16K CK 14CK
Crystal Oscillator, BOD
enabled
1 10 16K CK 14CK + 4.1ms
Crystal Oscillator, fast
rising power
1 11 16K CK 14CK + 65ms
Crystal Oscillator, slowly
rising power
Table 7-5. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
SUT1..0
Start-up Time from Power
Down and Power Save
Additional Delay from
Reset (V
CC
= 5.0V) Recommended usage
00 1K CK
(1)
4 ms
Fast rising power or BOD
enabled
01 1K CK
(1)
64 ms Slowly rising power
10 32K CK 64 ms Stable frequency at start-up
11 Reserved