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ATtiny26(L)
1477G–AVR–03/05
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 11. 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 fre-
quency 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% accuracy 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 section
“Calibration Byte” on page 109.
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 pro-
cess variations 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 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 11. Internal Calibrated RC Oscillator Operating Modes
CKSEL3..0 Nominal Frequency (MHz)
0001
(1)
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 (V
CC
= 5.0V) Recommended Usage
00 6 CK – BOD enabled
01 6 CK 4.1 ms Fast rising power
10
(1)
6 CK 65 ms Slowly rising power
11 Reserved
Bit 76543210
$31 ($51) 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 Specific Calibration Value
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