ATmega32A datasheet(32/352 Pages) ATMEL

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8155B–AVR–07/09

ATmega32A

8.10

8.10.1

OSCCAL – Oscillator Calibration Register

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

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

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

in the signature row High Byte (address 0x00) is automatically loaded into the OSCCAL Regis-

ter. 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 Oscil-

lator. Writing $FF 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 8.0 MHz. Tuning to other

values is not guaranteed, as indicated in Table 8-11.

Bit

765

432

CAL7

CAL6

CAL5

CAL4

CAL3

CAL2

CAL1

CAL0

OSCCAL

Read/Write

R/W

Initial Value

Device Specific Calibration Value

Table 8-11.

Internal RC Oscillator Frequency Range.

OSCCAL Value

Min Frequency in Percentage of

Nominal Frequency (%)

Max Frequency in Percentage of

Nominal Frequency (%)

$00

100

$7F

150

$FF

100

200

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