ATmega165 datasheet(28/336 Pages) ATMEL

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ATmega165/V

2573G–AVR–07/09

Oscillator Calibration Register

– OSCCAL

• Bits 6..0 – CAL6..0: Oscillator Calibration Value

Writing the calibration byte to this address will trim the internal Oscillator to remove pro-

cess variations from the Oscillator frequency. This is done automatically during Chip

Reset. When OSCCAL is zero, the lowest available frequency is chosen. Writing non-

zero values to this register will increase the frequency of the internal Oscillator. Writing

0x7F to the register gives the highest available frequency. The calibrated Oscillator is

used to time EEPROM and Flash access. If EEPROM or Flash is written, do not cali-

brate to more than 10% above the nominal frequency. Otherwise, the EEPROM or Flash

write may fail. Note that the Oscillator is intended for calibration to 8.0 MHz. Tuning to

other values is not guaranteed, as indicated in Table 10.

External Clock

To drive the device from an external clock source, XTAL1 should be driven as shown in

Figure 13. To run the device on an external clock, the CKSEL Fuses must be pro-

grammed to “0000”.

Figure 13. External Clock Drive Configuration

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

shown in Table 12.

Bit

7654

321

–

CAL6

CAL5

CAL4

CAL3

CAL2

CAL1

CAL0

OSCCAL

Read/Write

R/W

Initial Value

Device Specific Calibration Value

Table 10. Internal RC Oscillator Frequency Range.

OSCCAL Value

Min Frequency in Percentage of

Nominal Frequency

Max Frequency in Percentage of

Nominal Frequency

0x00

50%

100%

0x3F

75%

150%

0x7F

100%

200%

Table 11. Crystal Oscillator Clock Frequency

CKSEL3..0

Frequency Range

0000

0 - 16 MHz

EXTERNAL

CLOCK

SIGNAL

XTAL2

XTAL1

GND

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