Table 15. Start-up times for the internal calibrated RC Oscillator clock selection
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (VCC = 5.0V)
Power Conditions
BOD enabled
SUT1..0
00
6 CK
6 CK
14CK
Fast rising power
Slowly rising power
14CK + 4.1 ms
14CK + 65 ms(1)
01
6 CK
10
Reserved
11
Note:
1. The device is shipped with this option selected.
Oscillator Calibration Register
– OSCCAL
Bit
7
6
5
4
3
2
1
0
CAL7
R/W
CAL6
R/W
CAL5
R/W
CAL4
R/W
CAL3
R/W
CAL2
R/W
CAL1
R/W
CAL0
R/W
OSCCAL
Read/Write
Initial Value
Device Specific Calibration Value
• Bits 7..0 – CAL7..0: Oscillator Calibration Value
The Oscillator Calibration Register is used to trim the Calibrated Internal 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 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 7.3 - 8.1 MHz
within 1ꢀ accuracy. 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 cali-
brate to more than 8.8 MHz. Otherwise, the EEPROM or Flash write may fail.
The CAL7 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 frequency ranges are overlapping, in other words a setting of OSCCAL = 0x7F
gives a higher frequency than OSCCAL = 0x80.
The CAL6..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 CAL6..0 by 1 will give a frequency increment of
less than 2ꢀ in the frequency range 7.3 - 8.1 MHz.
46
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATMEL [ ATMEL ]
