ATTINY15 ATMEL [ATMEL Corporation], ATTINY15 Datasheet - Page 24

• Manufacturer Part Number: ATTINY15
• Manufacturer: ATMEL [ATMEL Corporation]

Tunable Internal RC
Oscillator
The System Clock Oscillator
Calibration Register –
OSCCAL
Internal PLL for Fast
Peripheral Clock
Generation
24
ATtiny15L
The internal RC oscillator provides a fixed 1.6 MHz clock (nominal at 5V and 25 ° C). This
internal clock is always the system clock of the ATtiny15L. This oscillator can be cali-
brated by writing the calibration byte (see page 55) to the OSCCAL register.
Writing the calibration byte to this address will trim the internal oscillator frequency in
order to remove process variations. When OSCCAL is zero (initial value), the lowest
available frequency is chosen. Writing non-zero values to this register will increase the
frequency of the internal oscillator. Writing $FF to the register selects the highest avail-
able frequency.
The internal PLL in ATtiny15L generates a clock frequency that is 16x multiplied from
the RC oscillator system clock. If the RC oscillator frequency is the nominal 1.6 MHz, the
fast peripheral clock is 25.6 MHz. The fast peripheral clock, or a clock prescaled from
that, can be selected as the clock source for Timer/Counter1.
The PLL is locked on the tunable internal RC oscillator and adjusting the tunable internal
RC oscillator via the OSCCAL register will adjust the fast peripheral clock at the same
time. Timer1 may malfunction if the internal RC oscillator is adjusted beyond 1.75 MHz.
It is recommended not to take the OSCCAL adjustments to a higher frequency than
1.75 MHz in order to keep proper operation of all chip functions.
Bit
$31
Read/Write
Initial Value
CAL7
R/W
7
0
CAL6
R/W
6
0
CAL5
R/W
5
0
CAL4
R/W
4
0
CAL3
R/W
3
0
CAL2
R/W
2
0
CAL1
R/W
1
0
CAL0
R/W
0
0
1187D–12/01
OSCCAL