Rainbow Electronics ATtiny43U - Manual (Page 25)

25
8048B–AVR–03/09
When this clock source is selected, start-up times are determined by SUT Fuses as shown in
Table 6-3 .
When applying an external clock, it is required to avoid sudden changes in the applied clock fre-
quency to ensure stable operation of the MCU. A variation in frequency of more than 2% from
one clock cycle to the next can lead to unpredictable behavior. If changes of more than 2% is
required, ensure that the MCU is kept in Reset during the changes.
Note that the System Clock Prescaler can be used to implement run-time changes of the internal
clock frequency while still ensuring stable operation. Refer to
“Power Management and Sleep
Modes” on page 31 for details.
6.2.2 Calibrated Internal 8 MHz Oscillator
By default, the Internal RC Oscillator provides an approximate 8.0 MHz clock. Though voltage
and temperature dependent, this clock can be very accurately calibrated by the user. See
Table
20-2 on page 157 for more details. The device is shipped with the CKDIV8 Fuse programmed.
See “System Clock Prescaler” on page 27 for more details.
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in
Table 6-4 . If selected, it will operate with no external components. During reset, hardware loads
the pre-programmed calibration value into the OSCCAL Register and thereby automatically cal-
ibrates the RC Oscillator. The accuracy of this calibration is shown as Factory calibration in
Table 20-2 on page 157 .
By changing the OSCCAL register from SW, see “OSCCAL – Oscillator Calibration Register” on
page 28 , it is possible to get a higher calibration accuracy than by using the factory calibration.
The accuracy of this calibration is shown as User calibration in Table 20-2 on page 157 .
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 cali-
bration value, see the section
“Calibration Byte” on page 142 .
Notes: 1. The device is shipped with this option selected.
Table 6-3. Start-up Times for the External Clock Selection
SUT1..0
Start-up Time
from Power-down
Additional Delay
from Reset Recommended Usage
00 6 CK 14CK BOD enabled
01 6 CK 14CK + 4 ms Fast rising power
10 6 CK 14CK + 64 ms Slowly rising power
11 Reserved
Table 6-4. Internal Calibrated RC Oscillator Operating Modes
CKSEL3..0 Nominal Frequency (MHz)
0010
(1)
8.0