ATMEGA88 datasheet(30/349 Pages) ATMEL

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ATmega48/88/168

2545D–AVR–07/04

Calibrated Internal RC

Oscillator

The calibrated internal RC Oscillator by default provides a 8.0 MHz clock. The frequency

is nominal value at 3V and 25

°C. The device is shipped with the CKDIV8 Fuse pro-

grammed. See “System Clock Prescaler” on page 33 for more details. This clock may

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

11. If selected, it will operate with no external components. During reset, hardware loads

the calibration byte into the OSCCAL Register and thereby automatically calibrates the

RC Oscillator. At 3V and 25

°C, this calibration gives a frequency of 8 MHz ± 1%. The

oscillator can be calibrated to any frequency in the range 7.3 - 8.1 MHz within ±1%

accuracy, by changing the OSCCAL register. 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 274.

Notes:

1. The device is shipped with this option selected.

2. The frequency ranges are preliminary values. Actual values are TBD.

3. 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 12 on page 30.

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.

Oscillator Calibration Register

– OSCCAL

• Bits 7..0 – CAL7..0: Oscillator Calibration Value

The Oscillator Calibration Register is used to trim the Calibrated Internal RC Oscillator

to remove process variations from the oscillator frequency. The factory-calibrated value

is automatically written to this register during chip reset, giving an oscillator frequency of

8.0 MHz at 25°C. The application software can write this register to change the oscillator

frequency. The oscillator can be calibrated to any frequency in the range 7.3 - 8.1 MHz

within ±1% accuracy. Calibration outside that range is not guaranteed.

Table 11. Internal Calibrated RC Oscillator Operating Modes

(1)(3)

Frequency Range(2) (MHz)

CKSEL3..0

7.3 - 8.1

0010

Table 12. 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 (V

CC = 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

Bit

7654

321

CAL7

CAL6

CAL5

CAL4

CAL3

CAL2

CAL1

CAL0

OSCCAL

Read/Write

R/W

Initial Value

Device Specific Calibration Value

ATMEGA88 Datasheet(PDF) 30 Page - ATMEL Corporation

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