attiny13-20ss ATMEL Corporation, attiny13-20ss Datasheet - Page 23

• Manufacturer Part Number: attiny13-20ss
• Description: 8-bit Microcontroller With 1k Bytes In-system Programmable Flash - Atmel Corporation
• Manufacturer: ATMEL Corporation

Calibrated Internal RC
Oscillator
Oscillator Calibration Register
– OSCCAL
2535G–AVR–01/07
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
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
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 106.
Table 4. 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 5..
Table 5. Start-up Times for the Internal Calibrated RC Oscillator Clock Selection
Note:
• 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
Bit
Read/Write
Initial Value
SUT1..0
10
00
01
11
(1)
1. The device is shipped with this option selected.
1. The device is shipped with this option selected.
CKSEL1..0
from Power-down
10
R
7
–
0
Start-up Time
01
(1)
6 CK
6 CK
6 CK
CAL6
R/W
6
CC
CAL5
R/W
), the CKDIV8 Fuse must be programmed in order to
5
Device Specific Calibration Value
Additional Delay from
CAL4
R/W
Reset (V
4
CC
14CK + 64 ms
14CK + 4 ms
Reserved
and Temperature. When this Oscillator is
14CK
CAL3
R/W
CC
3
= 5.0V)
Nominal Frequency
CAL2
R/W
2
9.6 MHz
4.8 MHz
Recommended Usage
BOD enabled
Fast rising power
Slowly rising power
CAL1
R/W
1
CAL0
R/W
0
OSCCAL
23