Rainbow-electronics Atmega169L Manuel d'utilisateur télécharger pdf (Page 27)

ATmega169V/L

2514A–AVR–08/02

Note: 1. These options should only be used if frequency stability at start-up is not important for

the application

Calibrated Internal RC

Oscillator

The calibrated internal RC Oscillator provides a fixed 8.0 MHz clock. The frequency is

nominal value at 3V and 25°C. If 8 MHz frequency exceeds the specification of the

device (depends on V

),theCKDIV8Fusemustbeprogrammedinordertodividethe

internal frequency by 8 during start-up. The device is shipped with the CKDIV8 Fuse

programmed. See “System Clock Prescaler” on page 30. for more details. This clock

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

Table 8. If selected, it will operate with no external components. During reset, hardware

loads the calibration byte into the OSCCAL Register and thereby automatically cali-

brates the RC Oscillator. At 3V and 25°C, this calibration gives a frequency within ± 1%

of the nominal frequency. 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 calibration value, see the section “Calibration Byte”

on page 268.

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 9.

Note: 1. The device is shipped with this option selected.

Table 7. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection

CKSEL3..0

Start-up Time from

Power-down and Power-save Recommended Usage

0100

(1)

1K CK

0101 32K CK Stable frequency at start-up

0110

(1)

1K CK

0111 32K CK Stable frequency at start-up

Table 8. Internal Calibrated RC Oscillator Operating Modes

(1)

CKSEL3..0 Nominal Frequency

0010 8.0 MHz

Table 9. Start-up times for the internal calibrated RC Oscillator clock selection

SUT1..0

Start-up Time from Power-

down and Power-save

Additional Delay from

Reset (V

= 5.0V) Recommended Usage

00 6 CK 14CK BOD enabled

01 6 CK 14CK + 4.1 ms Fast rising power

(1)

6 CK 14CK + 65 ms Slowly rising power

11 Reserved

1 2 ... 22 23 24 25 26 27 28 29 30 31 32 ... 316 317

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Rainbow-electronics Atmega169L Manuel d'utilisateur Page 27

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