Datasheet

8024A–AVR–04/08

ATmega8HVA/16HVA

The FCAL[4:0] 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 0x1F gives the highest frequency in the

range. Incrementing FCAL[4:0] by 1 will give a frequency increment of less than 1.5 % in the fre-

quency range 7.3-8.1 MHz. With an accurate time reference, an oscillator accuracy of ±1% can

be achieved after calibration. The frequency will drift with temperature, so run-time calibration

will be required to maintain the accuracy. Refer to ”OSI – Oscillator Sampling Interface” on page

28 for details.

The default FOSCCAL value found in the signature row, is selected such that it is in the the

lower half of a segment (see Figure 30-1 on page 174 for typical characteristics of the FAST RC

oscillator). It is therefore sufficient to use the default segment and the one below to calibrate the

frequency over the whole temperature range. To avoid a large frequency change when shifting

between the two segments, a FOSC SEGMENT value is stored in the signature row. This is the

first FOSCCAL value giving a lower frequency than the lowest value in the default segment, and

should be used when calibrating the Fast RC oscillator.

9.13.2 MCUCR – MCU Control Register

• Bit 5 – CKOE: Clock Output

When this bit is written to one, the CPU clock divided by 2 is output on the PB0 pin.

9.13.3 CLKPR – Clock Prescale Register

Bit 7 6 5 4 3 2 1 0

0x35 (0x55)

-– – CKOE PUD – – – – MCUCR

Read/Write R R R/W R/W R R R R

Initial Value 0 0 0 0 0 0 0 0

Bit 7 65432 1 0

(0x61) CLKPCE

– – – – – CLKPS1 CLKPS0 CLKPR

Read/WriteR/W RRRRRR/WR/W

Initial Value 0 00000 1 1