
29
1477K–AVR–08/10
ATtiny26(L)
Calibrated Internal
RC Oscillator
The calibrated internal RC Oscillator provides a fixed 1.0, 2.0, 4.0, or 8.0 MHz clock. All frequen-
cies are nominal values at 5V and 25
°C. This clock may be selected as the system clock by pro-
gramming the CKSEL Fuses as shown in
Table 11. If selected, it will operate with no external
components. The CKOPT Fuse should always be unprogrammed when using this clock option.
During Reset, hardware loads the calibration byte into the OSCCAL Register and thereby auto-
matically calibrates the RC Oscillator. At 5V, 25
°C and 1.0 MHz Oscillator frequency selected,
this calibration gives a fre-quency within ± 3% of the nominal frequency. Using run-time calibra-
tion methods as described in application notes available at www.atmel.com/avr it is possible to
achieve ± 1% accuracy at any given VCC 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 section
“Cali-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 12. PB4 (XTAL1) and PB5 (XTAL2) can be used as general I/O ports.
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 process vari-
ations from the oscillator frequency. During Reset, the 1 MHz calibration 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 value must be loaded manually.
This can be done by first reading the signature row by a programmer, and then store the calibra-
tion 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 will increase the frequency of the internal oscillator. Writing $FF
to the register gives the highest available frequency. The calibrated Oscillator is used to time
Table 11. Internal Calibrated RC Oscillator Operating Modes
CKSEL3..0
Nominal Frequency (MHz)
1.0
0010
2.0
0011
4.0
0100
8.0
Table 12. Start-up Times for the Internal Calibrated RC Oscillator Clock Selection
SUT1..0
Start-up Time from
Power-down
Additional Delay from
Reset (VCC = 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
76543210
CAL7
CAL6
CAL5
CAL4
CAL3
CAL2
CAL1
CAL0
OSCCAL
Read/Write
R/W
Initial Value
Device Specific Calibration Value