ATTINY26L8SU SL383 Atmel, ATTINY26L8SU SL383 Datasheet - Page 30

• Manufacturer Part Number: ATTINY26L8SU SL383
• Description: MCU 8-Bit ATtiny AVR RISC 2KB Flash 3.3V/5V 20-Pin SOIC T/R
• Manufacturer: Atmel

Calibrated Internal RC
Oscillator
Oscillator Calibration Register
– OSCCAL
30
ATtiny26(L)
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
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 111.
Table 11. Internal Calibrated RC Oscillator Operating Modes
Note:
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.
Table 12. Start-up Times for the Internal Calibrated RC Oscillator Clock Selection
Note:
• 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
SUT1..0
Bit
$31 ($51)
Read/Write
Initial Value
10
00
01
11
(1)
1. The device is shipped with this option selected.
1. The device is shipped with this option selected.
CKSEL3..0
Start-up Time from
0001
0010
0011
0100
CAL7
R/W
Power-down
7
(1)
6 CK
6 CK
6 CK
CAL6
R/W
6
CC
and Temperature. When this oscillator is used as the chip
CAL5
R/W
5
Device Specific Calibration Value
Additional Delay from
Reset (V
CAL4
R/W
4
4.1 ms
65 ms
Reserved
CC
–
CAL3
R/W
Nominal Frequency (MHz)
3
= 5.0V)
CAL2
R/W
2
1.0
2.0
4.0
8.0
Recommended Usage
BOD enabled
Fast rising power
Slowly rising power
CAL1
R/W
1
CAL0
R/W
0
1477J–AVR–06/07
OSCCAL