6 calibrated internal rc oscillator – Rainbow Electronics ATtiny861_V User Manual
Page 28
28
2588B–AVR–11/06
ATtiny261/461/861
7.6
Calibrated Internal RC 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
23-1 on page 188
and
”Internal Oscillator Speed” on page 211
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 on page 28
. 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 calibrates the RC Oscillator. The accuracy of this calibration is shown as Factory
calibration in
Table 23-1 on page 188
.
By changing the OSCCAL register from SW, see
”OSCCAL – Oscillator Calibration Register” on
, 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 23-1 on page 188
.
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 170
Notes:
1. The device is shipped with this option selected.
2. The frequency ranges are preliminary values. Actual values are TBD.
3. If 8 MHz frequency exceeds the specification of the device (depends on V
CC
), the CKDIV8
Fuse can be programmed in order to divide the internal frequency by 8.
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Table 7-7 on page 28
.
Note:
1. The device is shipped with this option selected.
Table 7-6.
Internal Calibrated RC Oscillator Operating Modes
(1)(3)
Frequency Range
(2)
(MHz)
CKSEL3..0
7.3 - 8.1
0010
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