ATTINY5_14 datasheet(21/170 Pages) ATMEL

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ATtiny4/5/9/10 [DATASHEET]

8127F–AVR–02/2013

6.5

6.5.1

CLKMSR – Clock Main Settings Register

• Bit 7:2 – Res: Reserved Bits

These bits are reserved and always read zero.

• Bit 1:0 – CLKMS[1:0]: Clock Main Select Bits

These bits select the main clock source of the system. The bits can be written at run-time to switch the source of

the main clock. The clock system ensures glitch free switching of the main clock source.

The main clock alternatives are shown in Table 6-3.

To avoid unintentional switching of main clock source, a protected change sequence must be followed to change

the CLKMS bits, as follows:

Write the signature for change enable of protected I/O register to register CCP

Within four instruction cycles, write the CLKMS bits with the desired value

6.5.2

OSCCAL – Oscillator Calibration Register

• Bits 7:0

– CAL[7:0]: Oscillator Calibration Value

The oscillator calibration register is used to trim the calibrated internal oscillator and 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 Table 16-2, “Calibration Accuracy of Internal RC

Oscillator,” on page 117.

The application software can write this register to change the oscillator frequency. The oscillator can be calibrated

to frequencies as specified in Table 16-2, “Calibration Accuracy of Internal RC Oscillator,” on page 117. Calibration

outside the range given is not guaranteed.

The CAL[7:0] bits are used to tune the frequency of the oscillator. A setting of 0x00 gives the lowest frequency, and

a setting of 0xFF gives the highest frequency.

Bit

765

432

0x37

–

CLKMS1

CLKMS0

CLKMSR

Read/Write

R/W

Initial Value

Table 6-3.

Selection of Main Clock

CLKM1

CLKM0

Main Clock Source

Calibrated Internal 8 MHzOscillator

Internal 128 kHz Oscillator (WDT Oscillator)

External clock

Reserved

Bit

765

432

0x39

CAL7

CAL6

CAL5

CAL4

CAL3

CAL2

CAL1

CAL0

OSCCAL

Read/Write

R/W

Initial Value

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