ATtiny2313 calibrated on-chip RC oscillator

The calibrated on-chip RC oscillator provides a fixed 8 MHz or 4 MHz clock. These frequencies are nominal values ​​at 3V and 25°C. If the frequency exceeds the device nominal value, the CKDIV8 fuse must be programmed to divide the internal frequency by 8 during the startup phase. See P24 "System Clock Prescaler" for details.

This clock can also be used as the system clock by programming the fuse CKSEL according to Table 4. No external components are required when this clock is selected. The hardware automatically calibrates the RC oscillator by loading the calibration byte into the OSCCAL register at reset. At 3V and 25°C, this calibration provides an accuracy of ± 10% of the nominal frequency. 

When using this oscillator as the system clock, the watchdog still uses its own watchdog timer as the basis for overflow reset. For more information on the calibration data, see P152 "Calibration Byte".

After selecting this oscillator, the startup time is determined by the fuse bit SUT, as shown in Table 7.

Oscillator Calibration Register - OSCCAL

· Bits 7 – RES: Reserved
bits, read returns zero.
· Bits 6..0 – CAL6..0: Oscillator calibration values
​​Writing calibration data to this address allows the internal oscillator to be adjusted to eliminate frequency deviations due to the production process. This is done automatically at reset. When OSCCAL is zero the oscillator operates at the minimum frequency. When writing non-zero data to it the frequency of the internal oscillator increases. Writing 0x7F gives the maximum frequency. The calibrated oscillator is used to time accesses to the EEPROM and Flash. Do not calibrate the frequency to more than 10% of the nominal frequency when writing to the EEPROM and Flash, otherwise the write operation may fail. Note that the oscillator is only calibrated for 8 MHz and 4 MHz frequencies, other frequencies are not guaranteed.

To ensure stable operation of the MCU, avoid drastically changing the nominal value when calibrating the internal RC oscillator. Sudden changes in the operating frequency exceeding 2% will produce abnormal behavior. Each change to the value in the OSCCAL register should not exceed 0x20.