Datasheet

29
7701F–AVR–10/12
Atmel ATtiny24/44/84 [Preliminary]
7.6 Calibrated Internal RC Oscillator
By default, the Internal RC Oscillator provides an approximate 8MHz clock. Though voltage
and temperature dependent, this clock can be very accurately calibrated by the user. See
Table 22-2 on page 180 and “Internal Oscillator Speed” on page 204 for more details. The
device is shipped with the CKDIV8 Fuse programmed. See “System Clock Prescaler” on page
31 for more details.
This clock may be selected as the system 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 pre-programmed calibration value into the OSCCAL Register and thereby automati-
cally calibrates the RC Oscillator. The accuracy of this calibration is shown as Factory
calibration in Table 22-2 on page 180.
By changing the OSCCAL register from SW, see “Oscillator Calibration Register – OSCCAL”
on page 32, it is possible to get higher calibration accuracy than by using the factory calibra-
tion. The accuracy of this calibration is shown as User calibration in Table 22-2 on page 180.
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 165.
Note: 1. The device is shipped with this option selected.
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Table 7-7 on page 29..
Note: 1. The device is shipped with this option selected.
7.7 External Clock
To drive the device from an external clock source, CLKI should be driven as shown in Figure
7-3 on page 30. To run the device on an external clock, the CKSEL Fuses must be pro-
grammed to “0000”.
Table 7-6. Internal Calibrated RC Oscillator Operating Modes
CKSEL3..0 Nominal Frequency
0010
(1)
8.0 MHz
Table 7-7. Start-up Times for the Internal Calibrated RC Oscillator Clock Selection
SUT1..0
Start-up Time
from Power-down
Additional Delay from
Reset (V
CC
= 5.0V) Recommended Usage
00 6 CK 14CK BOD enabled
01 6 CK 14CK + 4 ms Fast rising power
10
(1)
6 CK 14CK + 64 ms Slowly rising power
11 Reserved