ATA8741-PXQW【PDF 45/238 页】MCU W/TRANSMITTER ASK/FSK 24QFN

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Sheet目录52

41页 42页 43页 44页 [45页]46页 47页 48页 49页

9140CINDCO02/10

ATA8741

14.6 Calibrated Internal RC Oscillator

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 Table

29-2 on page 197 and Internal Oscillator Speed on page 221 for more details. The device is

shipped with the CKDIV8 Fuse programmed. See System Clock Prescaler on page 47 for

more details.

This clock may be selected as the system clock by programming the CKSEL Fuses as shown in

Table 14-6. If selected, it will operate with no external components. During reset, hardware loads

the pre-programmed calibration value into the OSCCAL Register and thereby automatically cal-

ibrates the RC Oscillator. The accuracy of this calibration is shown as Factory calibration in

Table 29-2 on page 197.

By changing the OSCCAL register from SW, see Oscillator Calibration Register OSCCAL on

page 48, 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 29-2 on page 197.

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 182.

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 14-7 on page 45..

Note:

1. The device is shipped with this option selected.

14.7 External Clock

To drive the device from an external clock source, CLKI should be driven as shown in Figure

14-3 on page 46. To run the device on an external clock, the CKSEL Fuses must be pro-

grammed to 0000.

Table 14-6. Internal Calibrated RC Oscillator Operating Modes

CKSEL3..0

Nominal Frequency

0010

(1)

8.0 MHz

Table 14-7. Start-up Times for the Internal Calibrated RC Oscillator Clock Selection

SUT1..0

Start-up Time

from Power-down

Additional Delay from

Reset (V

= 5.0V)

Recommended Usage

6 CK

14CK

BOD enabled

6 CK

14CK + 4 ms

Fast rising power

(1)

6 CK

14CK + 64 ms

Slowly rising power

Reserved

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