Datasheet

26
2586D–AVR–02/06
ATtiny25/45/85
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 Low-frequency Crystal Oscillator
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 7-3. 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 7-5.
Notes: 1. These options should only be used if frequency stability at start-up is not important for the
application.
7.6 Calibrated Internal RC Oscillator
The calibrated internal RC Oscillator provides an 8.0 MHz clock. The frequency is the nominal
value at 3V and 25°C. If the frequency exceeds the specification of the device (depends on V
CC
),
the CKDIV8 Fuse must be programmed in order to divide the internal frequency by 8 during
start-up. See Section “7.10” on page 30. for more details. This clock may be selected as the sys-
tem clock by programming the CKSEL Fuses as shown in Table 7-6. If selected, it will operate
with no external components. During reset, hardware loads the calibration byte into the OSC-
CAL Register and thereby automatically calibrates the RC Oscillator. At 3V and 25°C, this
calibration gives a frequency within ± 1% of the nominal frequency. When this Oscillator is used
as the chip clock, the Watchdog Oscillator will still be used for the Watchdog Timer and for the
Reset Time-out. For more information on the pre-programmed calibration value, see the section
”Calibration Byte” on page 152.
In addition the calibrated internal RC Oscillator provides a 6.4 MHz clock that is chosen by writ-
ing the CKSEL fuses to “0011” as shown in Table 7. When this CKSEL setting is written the
nominal frequency of the calibrated internal RC Oscillator is calibrated down to 6.4 MHz. This
clock frequency is needed for the ATtiny15 compatibility mode. In the ATtiny15 compatibility
mode the 6.4 MHz internal RC oscillator is used as a reference clock for the PLL that is generat-
Table 7-5. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
SUT1:0
Start-up Time from
Power Down
Additional Delay from
Reset (V
CC
= 5.0V) Recommended usage
00 1K (1024) CK
(1)
4 ms
Fast rising power or BOD
enabled
01 1K (1024) CK
(1)
64 ms Slowly rising power
10 32K (32768) CK 64 ms Stable frequency at start-up
11 Reserved