ATMEGA644 datasheet(35/374 Pages) ATMEL

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2593L–AVR–02/07

ATmega644

7.6.1

OSCCAL – Oscillator Calibration Register

• 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. A pre-programmed calibration value is

automatically written to this register during chip reset, giving the Factory calibrated frequency as

specified in Table 26-5 on page 323. The application software can write this register to change

the oscillator frequency. The oscillator can be calibrated to frequencies as specified in Table 26-

5 on page 323. Calibration outside that range is not guaranteed.

Note that this oscillator is used to time EEPROM and Flash write accesses, and these write

times will be affected accordingly. If the EEPROM or Flash are written, do not calibrate to more

than 8.8 MHz. Otherwise, the EEPROM or Flash write may fail.

The CAL7 bit determines the range of operation for the oscillator. Setting this bit to 0 gives the

lowest frequency range, setting this bit to 1 gives the highest frequency range. The two fre-

quency ranges are overlapping, in other words a setting of OSCCAL = 0x7F gives a higher

frequency than OSCCAL = 0x80.

The CAL6..0 bits are used to tune the frequency within the selected range. A setting of 0x00

gives the lowest frequency in that range, and a setting of 0x7F gives the highest frequency in the

range.

7.7

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 “0011” as shown in Table 7-10.

Note:

1. The frequency is preliminary value. Actual value is TBD.

When this clock source is selected, start-up times are determined by the SUT Fuses as shown in

Table 7-11.

Bit

543210

(0x66)

CAL7

CAL6

CAL5

CAL4

CAL3

CAL2

CAL1

CAL0

OSCCAL

Read/Write

R/W

Initial Value

Device Specific Calibration Value

Table 7-10.

128 kHz Internal Oscillator Operating Modes

Nominal Frequency

CKSEL3..0

128 kHz

0011

Table 7-11.

Start-up Times for the 128 kHz Internal Oscillator

Power Conditions

Start-up Time from Power-

down and Power-save

Additional Delay from

Reset

SUT1..0

BOD enabled

6 CK

14CK

Fast rising power

6 CK

14CK + 4 ms

Slowly rising power

6 CK

14CK + 64 ms

Reserved

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