Rainbow Electronics ATmega8HVD - Manual (Page 29)

29
8052B–AVR–09/08
ATmega4HVD/8HVD
8.13 Register Description
8.13.1 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
process variations from the oscillator frequency. The factory-calibrated value is automatically
written to this register during chip reset, giving an oscillator frequency of 8.0 MHz at 85
° C. The
application software can write this register to change the oscillator frequency. The oscillator
can be run-time calibrated to any frequency in the range 7.3-8.1 MHz. Calibration outside 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 8.1 MHz. Otherwise, the EEPROM or Flash write may fail.
The FCAL[7:5] bits determine the range of operation for the oscillator. Setting these bits to
0b000 gives the lowest frequency range, setting this bit to 0b111 gives the highest frequency
range. The frequency ranges are overlapping. A setting of for instance FOSCCAL = 0x1F
gives a higher frequency than FOSCCAL = 0x20.
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 frequency 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 Inter-
face” on page 27 for details.
8.13.2 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 PB2 pin.
8.13.3 CLKPR – Clock Prescale Register
Bit 7 6 5 4 3 2 1 0
FCAL7 FCAL6 FCAL5 FCAL4 FCAL3 FCAL2 FCAL1 FCAL0 FOSCCAL
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value Device Specific Calibration Value
Bit 7 6 5 4 3 2 1 0
-– – 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 6 5 4 3 2 1 0
CLKPCE – – – – – CLKPS1 CLKPS0 CLKPR
Read/Write R/W R R R R R R/W R/W
Initial Value 0 0 0 0 0 0 1 1