Datasheet

23
ATtiny13
2535E–AVR–10/04
Calibrated Internal RC
Oscillator
The calibrated internal RC Oscillator provides an 9.6 MHz or 4.8 MHz clock. The fre-
quency is the nominal value at 3V and 25°C. If the frequency exceeds the specification
of the device (depends on V
CC
), the CKDIV8 Fuse must be programmed in order to
divide the internal frequency by 8 during start-up. See “System Clock Prescaler” on
page 25. for more details. This clock may be selected as the system clock by program-
ming the CKSEL Fuses as shown in Table 4. If selected, it will operate with no external
components. During reset, hardware loads the calibration byte into the OSCCAL Regis-
ter and thereby automatically calibrates the RC Oscillator. At 3V and 25°C, this
calibration gives a frequency within ± 10% of the nominal frequency. Using calibration
methods as described in application notes available at www.atmel.com/avr it is possible
to achieve ± 3% 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 105.
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 5..
Note: 1. The device is shipped with this option selected.
Oscillator Calibration Register
– OSCCAL
• Bit 7 – Res: Reserved Bit
This bit is reserved bit in the ATtiny13 and will always read as zero.
• Bits 6..0 – CAL6..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. This is done automatically during Chip
Reset. 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
0x7F 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 cali-
brate to more than 10% above the nominal frequency. Otherwise, the EEPROM or Flash
Table 4. Internal Calibrated RC Oscillator Operating Modes
CKSEL1..0 Nominal Frequency
10
(1)
9.6 MHz
01 4.8 MHz
Table 5. 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 14CK BOD enabled
01 6 CK 14CK + 4 ms Fast rising power
10
(1)
6 CK 14CK + 64 ms Slowly rising power
11 Reserved
Bit 76543210
– CAL6 CAL5 CAL4 CAL3 CAL2 CAL1 CAL0 OSCCAL
Read/Write R R/W R/W R/W R/W R/W R/W R/W
Initial Value 0 Device Specific Calibration Value