ATmega328P datasheet(32/345 Pages) ATMEL

32 / 345 page

7810A–AVR–11/09

ATmega328P [Preliminary]

8.6

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

28-1 on page 307 for more details. The device is shipped with the CKDIV8 Fuse programmed.

See “System Clock Prescaler” on page 34 for more details.

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

Table 8-10. 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 28-1 on page 307.

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

page 35, 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 28-1 on page 307.

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 289.

Notes:

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

2. 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 8-11 on page 32.

Note:

1. If the RSTDISBL fuse is programmed, this start-up time will be increased to

14CK + 4.1 ms to ensure programming mode can be entered.

The device is shipped with this option selected.

8.7

128 kHz Internal Oscillator

The 128 kHz internal Oscillator is a low power Oscillator providing a clock of 128 kHz. The fre-

quency is nominal at 3V and 25°C. This clock may be select as the system clock by

programming the CKSEL Fuses to “11” as shown in Table 8-12..

Table 8-10.

Internal Calibrated RC Oscillator Operating Modes(1)(2)

Nominal Frequency (MHz)

CKSEL3..0

0010

Table 8-11.

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

Power Conditions

Start-up Time from

Power-down and Power-save

Additional Delay from

Reset (VCC = 5.0V)

SUT1..0

BOD enabled

6 CK

14CK(1)

Fast rising power

6 CK

14CK + 4.1 ms

Slowly rising power

6 CK

14CK + 65 ms(2)

Reserved

Table 8-12.

128 kHz Internal Oscillator Operating Modes

Nominal Frequency

CKSEL3..0

128 kHz

0011

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