ATMEGA406_0607 datasheet(29/263 Pages) ATMEL

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2548E–AVR–07/06

ATmega406

7.11

7.11.1

FOSCCAL – Fast RC Oscillator Calibration Register

• Bits 7:0 – FCAL7:0: Fast RC Oscillator Calibration Value

The Fast RC Oscillator Calibration Register is used to trim the Fast RC Oscillator to remove pro-

cess variations from the oscillator frequency. The factory-calibrated value is automatically

written to this register during chip reset, giving an oscillator frequency of 4.0 MHz at 25°C. The

application software can write this register to change the oscillator frequency. The oscillator can

be calibrated to any frequency in the range 3.7 - 4.0 MHz within ±1% accuracy. Calibration out-

side that range is not guaranteed.

Note that this oscillator is used to time EEPROM and Flash write accesses, and these write

times will be affected accordingly. If the EEPROM or Flash are written, do not calibrate to more

than 4.4 MHz. Otherwise, the EEPROM or Flash write may fail.

The FCAL7 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 fre-

quency ranges are overlapping, in other words a setting of FOSCCAL = 0x7F gives a higher

frequency than FOSCCAL = 0x80.

The FCAL6:0 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 FCAL6:0 by 1 will give a frequency increment of less than 2% in the fre-

quency range 3.7 - 4.0 MHz.

7.11.2

CCSR – Clock Control and Status Register

• Bits 7:2 - Res: Reserved Bits

These bits are reserved bits in the ATmega406 and will always read as zero.

• Bit 1 - XOE: 32 kHz Crystal Oscillator Enable

The XOE bit is used to enable the 32 kHz Crystal Oscillator before it is selected as clock source.

This allows the Oscillator clock to stabilize prior to use. The 32 kHz Crystal Oscillator requires

approximately two seconds to stabilize, this must be timed by the user software. If the software

tries to write a one to ACS and a zero to XOE at the same time, both XOE and ACS will be

cleared by the hardware. Thus, while the 32 kHz Crystal Oscillator is disabled it is not possible to

select it as a clock source .

Bit

543

210

(0x66)

FCAL7

FCAL6

FCAL5

FCAL4

FCAL3

FCAL2

FCAL1

FCAL0

FOSCCAL

Read/Write

R/W

Initial Value

Device Specific Calibration Value

Bit

3210

(0xC0)

–

–––

–

XOE

ACS

CCSR

Read/Write

R/W

Initial Value

0000

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