ATtiny25/45/85 Auto

6.6.1Oscillator Calibration Register – OSCCAL

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

Writing the calibration byte to this address will trim the internal Oscillator to remove process 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 0xFF 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 calibrate to more than 8.8 MHz frequency. 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 frequency 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. Incrementing CAL6..0 by 1 will give a frequency increment of less than 2% in the frequency range 7.3 - 8.1 MHz.

Avoid changing the calibration value in large steps when calibrating the calibrated internal RC Oscillator to ensure stable operation of the MCU. A variation in frequency of more than 2% from one cycle to the next can lead to unpredicatble behavior. Changes in OSCCAL should not exceed 0x20 for each calibration. It is required to ensure that the MCU is kept in Reset during such changes in the clock frequency

6.7External Clock

To drive the device from an external clock source, CLKI should be driven as shown in Figure 6- 4. To run the device on an external clock, the CKSEL Fuses must be programmed to “00”.

Figure 6-4. External Clock Drive Configuration

EXTERNAL

CLOCK CLKI

SIGNAL

GND

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7598C–AVR–09/06