ATMEGA261 View Datasheet(PDF) - Atmel Corporation
Part Name
Description
MFG CO.
'ATMEGA261' PDF : 226 Pages View PDF
ATtiny261/ATtiny461/ATtiny861
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 189 and “Internal Oscillator Speed” on page 207 for more details. The device is
shipped with the CKDIV8 Fuse programmed. See “System Clock Prescaler” on page 32 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 29. 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 189.
By changing the OSCCAL register from SW, see “OSCCAL – Oscillator Calibration Register” on
page 32, 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 189.
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 173.
Table 7-6.
Internal Calibrated RC Oscillator Operating Modes(1)(2)
Frequency Range (MHz)
7.84 - 8.16
CKSEL3..0
0010
Notes: 1. The device is shipped with this option selected.
2. If 8 MHz frequency exceeds the specification of the device (depends on VCC), 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 29.
Table 7-7.
SUT1..0
00
01
10(1)
11
Start-up Times for the Internal Calibrated RC Oscillator Clock Selection
Start-up Time
from Power-down
6 CK
Additional Delay from
Reset (VCC = 5.0V)
14CK
Recommended Usage
BOD enabled
6 CK
14CK + 4 ms
Fast rising power
6 CK
14CK + 64 ms
Slowly rising power
Reserved
Note: 1. The device is shipped with this option selected.
29
7753E–AVR–06/10
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