36
ATmega8515(L)
2512A–AVR–04/02
Calibrated Internal RC
Oscillator
The calibrated internal RCOscillatorprovides a fixed 1.0, 2.0, 4.0, or 8.0 MHz clock.All
frequencies are nominal values at 5V and 25°C.Thisclock maybe selected as the sys-
tem clock by programming the CKSEL Fuses asshowninTable12.If selected, it will
operate with no externalcomponents.The CKOPT Fuse should always beunpro-
grammedwhen using thisclock option. During reset,hardware loads the calibration byte
into the OSCCAL Register and thereby automatically calibrates theRCOscillator.At5V,
25°C, and 1.0 MHz Oscillatorfrequency selected, thiscalibration gives a frequency
within ± 1% of the nominalfrequency. When thisOscillator is used as the chipclock, the
Watchdog Oscillatorwill still beusedfor the Watchdog Timer andfor theReset Time-
out. For moreinformation on the pre-programmedcalibration value,see the section
“Calibration Byte” on page 177.
Note: 1. The deviceisshippedwith this option selected.
When thisOscillator isselected, start-up times are determinedbythe SUT Fuses as
showninTable13. XTAL1andXTAL2 should be leftunconnected(NC).
Note: 1. The deviceisshippedwith this option selected.
Oscillator Calibration Register
– OSCCAL
• Bits 7..0 – CAL7..0: Oscillator Calibration Value
Writing the calibration byte to this address will trim theinternal oscillator to remove pro-
cess variationsfrom the Oscillatorfrequency.This isdone automatically during Chip
Reset. When OSCCAL iszero, the lowestavailable frequency ischosen. Writing non-
zerovalues to thisregisterwill increasethe frequency of theinternalOscillator. Writing
$FF to the register gives the highestavailable frequency.The calibratedOscillator is
used to time EEPROM andFlash access. If EEPROM orFlash iswritten,donotcali-
bratetomorethan 10% above the nominalfrequency. Otherwise, the EEPROM orFlash
write mayfail. Note that the Oscillator is intendedforcalibrationto1.0, 2.0, 4.0, or
8.0 MHz. Tuning to other values is not guaranteed, as indicated in Table 14.
Table 12. InternalCalibrated RCOscillatorOperating Modes
CKSEL3..0 Nominal Frequency (MHz)
0001
(1)
1.0
0010 2.0
0011 4.0
0100 8.0
Table 13. Start-up Timesfor the InternalCalibrated RCOscillatorClock Selection
SUT1..0
Start-up Time from
Power-down
Additional Delay from
Reset (V
CC
= 5.0V) Recommended Usage
00 6CK – BOD enabled
01 6CK 4.1 msFast rising power
10
(1)
6CK 65msSlowly rising power
11 Reserved
Bit 76543 210
CAL7 CAL6 CAL5 CAL4 CAL3 CAL2 CAL1 CAL0 OSCCAL
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value Device SpecificCalibration Value
(24 Seiten)
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