Datasheet

13.9.1 OSCCAL – The Oscillator Calibration Register

Name: OSCCAL

Offset: 0x31 [ID-00000e1c]

Reset: 0x00

Property: When addressing I/O Registers as data space the offset address is 0x51

When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When

addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset

addresses.

Bit 7 6 5 4 3 2 1 0

CALn[7:0]

Access

R/W R/W R/W R/W R/W R/W R/W R/W

Reset x x x x x x x x

Bits 7:0 – CALn[7:0] Oscillator Calibration Value [n = 7:0]

Writing the calibration byte to this address will trim the Internal Oscillator to remove process variations

from the Oscillator frequency. During Reset, the 1MHz calibration value which is located in the signature

row High byte (address 0x00) is automatically loaded into the OSCCAL Register. If the internal RC is

used at other frequencies, the calibration values must be loaded manually. This can be done by first

reading the signature row by a programmer, and then store the calibration values in the Flash or

EEPROM. Then the value can be read by software and loaded into the OSCCAL Register. 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 10% above the nominal frequency. Otherwise, the EEPROM or

Flash write may fail. Note that the Oscillator is intended for calibration to 1.0, 2.0, 4.0, or 8.0MHz. Tuning

to other values is not guaranteed, as indicated in the following table.

Note: The OSCCAL reset value is the device specific calibration value.

Table 13-11. Internal RC Oscillator Frequency Range

OSCCAL Value Min Frequency in Percentage of

Nominal Frequency (%)

Max Frequency in Percentage of

Nominal Frequency (%)

0x00 50 100

0x7F 75 150

0xFF 100 200

AVR 8-Bit Microcontroller

System Clock and Clock Options

Datasheet Complete

40001974A-page 53