attiny25v ATMEL Corporation, attiny25v Datasheet - Page 32

• Manufacturer Part Number: attiny25v
• Description: Microcontroller With 2/4/8k Bytes In-system Programmable Flash
• Manufacturer: ATMEL Corporation

6.4
6.5
6.5.1
32
Clock Output Buffer
Register Description
ATtiny25/45/85
OSCCAL – Oscillator Calibration Register
The device can output the system clock on the CLKO pin (when not used as XTAL2 pin). To
enable the output, the CKOUT Fuse has to be programmed. This mode is suitable when the chip
clock is used to drive other circuits on the system. Note that the clock will not be output during
reset and that the normal operation of the I/O pin will be overridden when the fuse is pro-
grammed. Any clock source, including the internal RC Oscillator, can be selected when the clock
is output on CLKO. If the System Clock Prescaler is used, it is the divided system clock that is
output.
• Bits 7:0 – CAL7:0: Oscillator Calibration Value
The Oscillator Calibration Register is used to trim the Calibrated Internal RC Oscillator to
remove process variations from the oscillator frequency. A pre-programmed calibration value is
automatically written to this register during chip reset, giving the Factory calibrated frequency as
specified in
the oscillator frequency. The oscillator can be calibrated to frequencies as specified in
2 on page
Note that this oscillator is used to time EEPROM and Flash write accesses, and these write
times will be affected accordingly. If the EEPROM or Flash are written, do not calibrate to more
than 8.8 MHz. Otherwise, the EEPROM or Flash write may fail.
The CAL7 bit determines the range of operation for the oscillator. Setting this bit to 0 gives the
lowest frequency range, setting this bit to 1 gives the highest frequency range. The two fre-
quency ranges are overlapping, in other words a setting of OSCCAL = 0x7F gives a higher
frequency than OSCCAL = 0x80.
The CAL6..0 bits are used to tune the frequency within the selected range. A setting of 0x00
gives the lowest frequency in that range, and a setting of 0x7F gives the highest frequency in the
range.
To ensure stable operation of the MCU the calibration value should be changed in small. A vari-
ation in frequency of more than 2% from one cycle to the next can lead to unpredicatble
behavior. Changes in OSCCAL should not exceed 0x20 for each calibration. It is required to
ensure that the MCU is kept in Reset during such changes in the clock frequency
Table 6-13.
Bit
0x31
Read/Write
Initial Value
OSCCAL Value
0x3F
0x7F
0x00
169. Calibration outside that range is not guaranteed.
Table 21-2 on page
7
CAL7
R/W
Device Specific Calibration Value
Internal RC Oscillator Frequency Range
with Respect to Nominal Frequency
6
CAL6
R/W
Typical Lowest Frequency
5
CAL5
R/W
169. The application software can write this register to change
100%
50%
75%
4
CAL4
R/W
3
CAL3
R/W
with Respect to Nominal Frequency
2
CAL2
R/W
Typical Highest Frequency
1
CAL1
R/W
100%
150%
200%
CAL0
0
R/W
2586K–AVR–01/08
Table 21-
OSCCAL