Calibrated Internal Rc Oscillator - Atmel ATtiny25 Manual

6.6

Calibrated Internal RC Oscillator

6.6.1 Oscillator Calibration Register – OSCCAL
ATtiny25/45/85
26
The calibrated internal RC Oscillator provides an 8.0 MHz clock. The frequency is the nominal
value at 3V and 25°C. If the frequency exceeds the specification of the device (depends on V
the CKDIV8 Fuse must be programmed in order to divide the internal frequency by 8 during
start-up. See "System Clock Prescaler" on page 29.
as the system clock by programming the CKSEL Fuses as shown in
operate with no external components. During reset, hardware loads the calibration byte into the
OSCCAL Register and thereby automatically calibrates the RC Oscillator. At 3V and 25°C, this
calibration gives a frequency within ± 1% of the nominal frequency. 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
Table 6-6. Internal Calibrated RC Oscillator Operating Modes
CKSEL3..0
(1)
0010
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 6-7.
Table 6-7. Start-up Times for the Internal Calibrated RC Oscillator Clock Selection
Start-up Time
SUT1..0 from Power-down
00
01
(1)
10
11
Note: 1. The device is shipped with this option selected.
Bit 7
CAL7
Read/Write R/W
Initial Value
• 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. This is done automatically during Chip Reset. When
OSCCAL is zero, the lowest available frequency is chosen. Writing non-zero values to this regis-
ter will increase the frequency of the internal Oscillator. Writing 0xFF to the register gives the
highest available frequency. The calibrated Oscillator is used to time EEPROM and Flash
access. If EEPROM or Flash is written, do not calibrate to more than 8.8 MHz frequency. Other-
wise, the EEPROM or Flash write may fail.
137.
Additional Delay from
Reset (V
6 CK
6 CK
6 CK
6 5 4
CAL6 CAL5 CAL4
R/W R/W R/W
Device Specific Calibration Value
for more details. This clock may be selected
Table
Nominal Frequency
8.0 MHz
= 5.0V) Recommended Usage
CC
14CK + 4 ms BOD enabled
14CK + 4 ms Fast rising power
14CK + 64 ms Slowly rising power
Reserved
3 2 1
CAL3 CAL2 CAL1
R/W R/W R/W
),
CC
6-6. If selected, it will
0
CAL0 OSCCAL
R/W
7598H–AVR–07/09