39
ATmega8515(L)
2512K–AVR–01/10
Calibrated Internal RC
Oscillator
The calibrated internal RC Oscillator provides a fixed 1.0, 2.0, 4.0, or 8.0 MHz clock. All
frequencies are nominal values at 5V and 25
°C. This clock may be selected as the sys-
tem clock by programming the CKSEL Fuses as shown in
Table 12. If selected, it will
operate with no external components. The CKOPT Fuse should always be unpro-
grammed when using this clock option. During reset, hardware loads the calibration byte
into the OSCCAL Register and thereby automatically calibrates the RC Oscillator. At 5V,
25
°C, and 1.0 MHz Oscillator frequency selected, this calibration gives a frequency
within ± 3% of the nominal frequency. Using run-time calibration methods as described
in application notes available at www.atmel.com/avr it is possible to achieve ± 1% accu-
racy at any given V
CC and Temperature. When this Oscillator is used as the chip clock,
the Watchdog Oscillator will still be used for the Watchdog Timer and for the Reset
Time-out. For more information on the pre-programmed calibration value, see the sec-
Note:
1. The device is shipped with this option selected.
When this Oscillator is selected, start-up times are determined by the SUT Fuses as
shown in
Table 13. XTAL1 and XTAL2 should be left unconnected (NC).
Note:
1. The device is shipped with this option selected.
Oscillator Calibration Register
– OSCCAL
Bits 7..0 – CAL7..0: Oscillator Calibration Value
Writing the calibration byte to this address will trim the internal Oscillator to remove pro-
cess variations from the Oscillator frequency. During Reset, the 1 MHz calibrated value
which is located in the signature row High Byte (address 0x00) is automatically loaded
into the OSCCAL Register. If the internal RC is used at other frequencies, the calibration
values must be loaded manually. This can be done by first reading the signature row by
a programmer, and then store the calibration values in the Flash or EEPROM. Then the
value can be read by software and loaded into the OSCCAL Register. When OSCCAL is
zero, the lowest available frequency is chosen. Writing non-zero values to this register
Table 12. Internal Calibrated RC Oscillator Operating Modes
CKSEL3..0
Nominal Frequency (MHz)
1.0
0010
2.0
0011
4.0
0100
8.0
Table 13. Start-up Times for the Internal Calibrated RC Oscillator Clock Selection
SUT1..0
Start-up Time from
Power-down
Additional Delay from
Reset (V
CC = 5.0V)
Recommended Usage
00
6 CK
–
BOD enabled
01
6 CK
4.1 ms
Fast rising power
6 CK
65 ms
Slowly rising power
11
Reserved
Bit
7654
321
0
CAL7
CAL6
CAL5
CAL4
CAL3
CAL2
CAL1
CAL0
OSCCAL
Read/Write
R/W
Initial Value
Device Specific Calibration Value