ATA8741 ATMEL Corporation, ATA8741 Datasheet - Page 45

• Manufacturer Part Number: ATA8741
• Manufacturer: ATMEL Corporation

14.6
14.7
9140B–INDCO–07/09
Calibrated Internal RC Oscillator
External Clock
By default, the Internal RC Oscillator provides an approximate 8 MHz clock. Though voltage and
temperature dependent, this clock can be very accurately calibrated by the the user. See
29-2 on page 197
shipped with the CKDIV8 Fuse programmed. See
more details.
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in
Table
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 29-2 on page
By changing the OSCCAL register from SW, see
page
The accuracy of this calibration is shown as User calibration in
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
Table 14-6.
Note:
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Table 14-7 on page
Table 14-7.
Note:
To drive the device from an external clock source, CLKI should be driven as shown in
14-3 on page
grammed to “0000”.
SUT1..0
10
00
01
11
48, it is possible to get a higher calibration accuracy than by using the factory calibration.
(1)
14-6. If selected, it will operate with no external components. During reset, hardware loads
1. The device is shipped with this option selected.
1. The device is shipped with this option selected.
CKSEL3..0
Internal Calibrated RC Oscillator Operating Modes
Start-up Times for the Internal Calibrated RC Oscillator Clock Selection
46. To run the device on an external clock, the CKSEL Fuses must be pro-
0010
from Power-down
and
(1)
197.
45..
Start-up Time
“Internal Oscillator Speed” on page 221
6 CK
6 CK
6 CK
“Calibration Byte” on page
Additional Delay from
Reset (V
Reserved
“Oscillator Calibration Register – OSCCAL” on
14CK + 64 ms
14CK + 4 ms
“System Clock Prescaler” on page 47
14CK
CC
182.
Nominal Frequency
= 5.0V)
8.0 MHz
Table 29-2 on page
for more details. The device is
Recommended Usage
BOD enabled
Fast rising power
Slowly rising power
ATA8741
197.
Figure
Table
for
45